## Notations

The below are the simplest possible descriptions of various symbols, just to help you keep reading if you do not remember/know what they stand for.

Watch it. There are so many ad hoc usages of symbols, some will have been overlooked here. Always use common sense first in guessing what a symbol means in a given context.

A dot might indicate
• A dot product between vectors, if in between them.
• A time derivative of a quantity, if on top of it.
And also many more prosaic things (punctuation signs, decimal points, ...).

Multiplication symbol. May indicate:
• An emphatic multiplication.
• Multiplication continued on the next line / from the previous line.
• A vectorial product between vectors. In index notation, the -th component of equals

where is the index following in the sequence 123123..., and the one preceding it (or second following). Alternatively, evaluate the determinant

Might be used to indicate a factorial. Example: .

The function that generalizes to noninteger values of is called the gamma function; . The gamma function generalization is due to, who else, Euler. (However, the fact that instead of is due to the idiocy of Legendre.) In Legendre-resistant notation,

Straightforward integration shows that is 1 as it should, and integration by parts shows that , which ensures that the integral also produces the correct value of for any higher integer value of than 0. The integral, however, exists for any real value of above , not just integers. The values of the integral are always positive, tending to positive infinity for both , (because the integral then blows up at small values of ), and for , (because the integral then blows up at medium-large values of ). In particular, Stirling’s formula says that for large positive , can be approximated as

where the value indicated by the dots becomes negligibly small for large . The function can be extended further to any complex value of , except the negative integer values of , where is infinite, but is then no longer positive. Euler’s integral can be done for by making the change of variables , producing the integral , or , which equals and the integral under the square root can be done analytically using polar coordinates. The result is that

To get , multiply by , since .

May indicate:
• The magnitude or absolute value of the number or vector, if enclosed between a pair of them.
• The determinant of a matrix, if enclosed between a pair of them.
• The norm of the function, if enclosed between two pairs of them.

Summation symbol. Example: if in three dimensional space a vector has components , , , then stands for .

Integration symbol, the continuous version of the summation symbol. For example,

is the summation of over all little fragments that make up the entire -range.

May indicate:
• An approaching process. indicates for practical purposes the value of the expression following the when is extremely small. Similarly, indicates the value of the following expression when is extremely large.
• The fact that the left side leads to, or implies, the right-hand side.

Vector symbol. An arrow above a letter indicates it is a vector. A vector is a quantity that requires more than one number to be characterized. Typical vectors in physics include position , velocity , linear momentum , acceleration , force , moment , etcetera.

May indicate:
• A derivative of a function. Examples: , , , , .
• A small or modified quantity.

The spatial differentiation operator nabla. In Cartesian coordinates:

Nabla can be applied to a scalar function in which case it gives a vector of partial derivatives called the gradient of the function:

Nabla can be applied to a vector in a dot product multiplication, in which case it gives a scalar function called the divergence of the vector:

or in index notation

Nabla can also be applied to a vector in a vectorial product multiplication, in which case it gives a vector function called the curl or rot of the vector. In index notation, the -th component of this vector is

where is the index following in the sequence 123123..., and the one preceding it (or the second following it).

The operator is called the Laplacian. In Cartesian coordinates:

In non Cartesian coordinates, don’t guess; look these operators up in a table book.

A superscript star normally indicates a complex conjugate. In the complex conjugate of a number, every is changed into a .

Less than.

Greater than.

Emphatic equals sign. Typically means “by definition equal” or “everywhere equal.”

Indicates approximately equal. Normally the approximation applies when something is small or large. Read it as “is approximately equal to.”

Proportional to. The two sides are equal except for some unknown constant factor.

(Gamma) May indicate:
• The Gamma function. Look under “!” for details.

(capital delta) May indicate:
• An increment in the quantity following it.
• Often used to indicate the Laplacian .

(delta) May indicate:
• With two subscripts, the “Kronecker delta”, which by definition is equal to one if its two subscripts are equal, and zero in all other cases.

• Without two subscripts, the “Dirac delta function”, which is infinite when its argument is zero, and zero if it is not. In addition the infinity is such that the integral of the delta function over its single nonzero point is unity. The delta function is not a normal function, but a distribution. It is best to think of it as the approximate function shown in the right hand side of figure 2.5 for a very, very, small positive value of .

One often important way to create a three-dimensional delta function in spherical coordinates is to take the Laplacian of the function . In two dimensions, take the Laplacian of to get a delta function.

• Often used to indicate a small amount of the following quantity, or of a small change in the following quantity. There are nuanced differences in the usage of , and that are too much to go in here.

• Often used to indicate a second small quantity in addition to .

(partial) Indicates a vanishingly small change or interval of the following variable. For example, is the ratio of a vanishingly small change in function divided by the vanishingly small change in variable that causes this change in . Such ratios define derivatives, in this case the partial derivative of with respect to .

(variant of epsilon) May indicate:
• A very small quantity.

(eta) May be used to indicate a -position.

(capital theta) Used in this book to indicate some function of to be determined.

(theta) May indicate:
• In spherical coordinates, the angle from the chosen axis, with apex at the origin.
• a -position.
• A generic angle, like the one between the vectors in a cross or dot product.

(variant of theta) An alternate symbol for .

(lambda) May indicate:
• Wave length.
• An eigenvalue.
• Some multiple of something.

(xi) May indicate:
• An -position.

(pi) May indicate:
• The area of a circle of unit radius. Value 3.141592...
• Half the perimeter of a circle of unit radius. Value 3.141592...
• A 180 angle expressed in radians. Note that . Value 3.141592...

(rho) May indicate:

(tau) May indicate:
• A time or time interval.

(capital phi) May indicate:
• Some function of to be determined.

(phi) May indicate:
• In spherical coordinates, the angle around the chosen axis. Increasing by encircles the -axis exactly once.
• A phase angle.
• Something equivalent to an angle.

(variant of phi) May indicate:
• A change in angle .
• An alternate symbol for .

(omega) May indicate:
• Angular frequency.

May indicate:
• Some generic matrix or operator.
• Some constant.
• Area.

May indicate:
• Acceleration.
• Start point of an integration interval.
• Some coefficient.
• Some constant.

absolute
May indicate:
• The absolute value of a real number is indicated by . It equals is is positive or zero and if is negative.
• The absolute value of a complex number is indicated by . It equals the length of the number plotted as a vector in the complex plane. This simplifies to above definition if is real.

The adjoint or of a matrix is the complex-conjugate transpose of the matrix.

Alternatively, it is the matrix you get if you take it to the other side of an inner product. (While keeping the value of the inner product the same regardless of whatever two vectors or functions may be involved.)

Unitary”matrices are the inverse of their adjoint. Unitary matrices generalize rotations and reflections of vectors. Unitary operators preserve inner products.

Fourier transforms are unitary operators on account of the Parseval equality that says that inner products are preserved.

angle
According to trigonometry, if the length of a segment of a circle is divided by its radius, it gives the total angular extent of the circle segment. More precisely, it gives the angle, in radians, between the line from the center to the start of the circle segment and the line from the center to the end of the segment. The generalization to three dimensions is called the “solid angle;” the total solid angle over which a segment of a spherical surface extends, measured from the center of the sphere, is the area of that segment divided by the square radius of the sphere.

May indicate:
• A generic second matrix.
• Some constant.

May indicate:
• End point of an integration interval.
• Some coefficient.
• Some constant.

basis
A basis is a minimal set of vectors or functions that you can write all other vectors or functions in terms of. For example, the unit vectors , , and are a basis for normal three-dimensional space. Every three-dimensional vector can be written as a linear combination of the three.

May indicate:
• A third matrix.
• A constant.

Cauchy-Schwartz inequality
The Cauchy-Schwartz inequality describes a limitation on the magnitude of inner products. In particular, it says that for any vectors and vec

For example, if and are real vectors, the inner product is the dot product and we have

where is the length of vector and the one of , and is the angle in between the two vectors. Since a cosine is less than one in magnitude, the Cauchy-Schwartz inequality is therefore true for vectors.

The cosine function, a periodic function oscillating between 1 and -1 as shown in [2, pp. 40-...].

curl
The curl of a vector field is defined as .

Indicates a vanishingly small change or interval of the following variable. For example, can be thought of as a small segment of the -axis.

derivative
A derivative of a function is the ratio of a vanishingly small change in a function divided by the vanishingly small change in the independent variable that causes the change in the function. The derivative of with respect to is written as , or also simply as . Note that the derivative of function is again a function of : a ratio can be found at every point . The derivative of a function with respect to is written as to indicate that there are other variables, and , that do not vary.

determinant
The determinant of a square matrix is a single number indicated by . If this number is nonzero, can be any vector for the right choice of . Conversely, if the determinant is zero, can only produce a very limited set of vectors. But if it can produce a vector , it can do so for multiple vectors .

There is a recursive algorithm that allows you to compute determinants from increasingly bigger matrices in terms of determinants of smaller matrices. For a matrix consisting of a single number, the determinant is simply that number:

(This determinant should not be confused with the absolute value of the number, which is written the same way. Since we normally do not deal with matrices, there is normally no confusion.) For matrices, the determinant can be written in terms of determinants:

so the determinant is in short. For matrices, we have

and we already know how to work out those determinants, so we now know how to do determinants. Written out fully:

For determinants,

Etcetera. Note the alternating sign pattern of the terms.

As you might infer from the above, computing a good size determinant takes a large amount of work. Fortunately, it is possible to simplify the matrix to put zeros in suitable locations, and that can cut down the work of finding the determinant greatly. We are allowed to use the following manipulations without seriously affecting the computed determinant:

1. We may “transpose”the matrix, i.e. change its columns into its rows.
2. We can create zeros in a row by subtracting a suitable multiple of another row.
3. We may also swap rows, as long as we remember that each time that we swap two rows, it will flip over the sign of the computed determinant.
4. We can also multiply an entire row by a constant, but that will multiply the computed determinant by the same constant.
Applying these tricks in a systematic way, called “Gaussian elimination” or “reduction to lower triangular form”, we can eliminate all matrix coefficients for which is greater than , and that makes evaluating the determinant pretty much trivial.

div(ergence)
The divergence of a vector field is defined as .

May indicate:
• The basis for the natural logarithms. Equal to 2.71281828459... This number produces the “exponential function” , or , or in words “ to the power ”, whose derivative with respect to is again . If is a constant, then the derivative of is . Also, if is an ordinary real number, then is a complex number with magnitude 1.

Assuming that is an ordinary real number, and a real variable, is a complex function of magnitude one. The derivative of with respect to is

eigenvector
A vector is an eigenvector of a matrix if is nonzero and for some number called the corresponding eigenvalue.

exponential function
A function of the form , also written as . See function and .

May indicate:
• The anti-derivative of some function .
• Some function.

May indicate:
• A generic function.
• A fraction.
• Frequency.

function
A mathematical object that associates values with other values. A function associates every value of with a value . For example, the function associates with , with , with , with , with , and more generally, any arbitrary value of with the square of that value . Similarly, function associates any arbitrary with its cube , associates any arbitrary with the sine of that value, etcetera.

One way of thinking of a function is as a procedure that allows you, whenever given a number, to compute another number.

functional
A functional associates entire functions with single numbers. For example, the expectation energy is mathematically a functional: it associates any arbitrary wave function with a number: the value of the expectation energy if physics is described by that wave function.

May indicate:
• A second generic function.

The gradient of a scalar is defined as .

The imaginary part of a complex number. If with and real numbers, then . Note that .

May indicate:
• The number of a particle.
• A summation index.
• A generic index or counter.
Not to be confused with .

The standard square root of minus one: , , , .

index notation
A more concise and powerful way of writing vector and matrix components by using a numerical index to indicate the components. For Cartesian coordinates, we might number the coordinates as 1, as 2, and as 3. In that case, a sum like can be more concisely written as . And a statement like can be more compactly written as . To really see how it simplifies the notations, have a look at the matrix entry. (And that one shows only 2 by 2 matrices. Just imagine 100 by 100 matrices.)

iff
Emphatic “if.” Should be read as “if and only if.”

integer
Integer numbers are the whole numbers: .

inverse
(Of matrices.) If a matrix converts a vector into a vector , then the inverse of the matrix, , converts back into .

in other words, with the unit, or identity, matrix.

The inverse of a matrix only exists if the matrix is square and has nonzero determinant.

May indicate:
• A summation index.
• A generic index or counter.

May indicate:
• A generic summation index.

May indicate:
• The azimuthal quantum number.
• A generic summation index.

May indicate:
• A length.

Indicates the final result of an approaching process. indicates for practical purposes the value of the following expression when is extremely small.

linear combination
A very generic concept indicating sums of objects times coefficients. For example, a position vector is the linear combination with the objects the unit vectors , , and and the coefficients the position coordinates , , and .

matrix
A table of numbers.

As a simple example, a two-dimensional matrix is a table of four numbers called , , , and :

unlike a two-dimensional (ket) vector , which would consist of only two numbers and arranged in a column:

(Such a vector can be seen as a “rectangular matrix” of size , but let’s not get into that.)

In index notation, a matrix is a set of numbers indexed by two indices. The first index is the row number, the second index is the column number. A matrix turns a vector into another vector according to the recipe

where stands for “the -th component of vector ,” and for “the -th component of vector .”

As an example, the product of and above is by definition

which is another two-dimensional ket vector.

Note that in matrix multiplications like the example above, in geometric terms we take dot products between the rows of the first factor and the column of the second factor.

To multiply two matrices together, just think of the columns of the second matrix as separate vectors. For example:

which is another two-dimensional matrix. In index notation, the component of the product matrix has value .

The zero matrix is like the number zero; it does not change a matrix it is added to and turns whatever it is multiplied with into zero. A zero matrix is zero everywhere. In two dimensions:

A unit matrix is the equivalent of the number one for matrices; it does not change the quantity it is multiplied with. A unit matrix is one on its “main diagonal” and zero elsewhere. The 2 by 2 unit matrix is:

More generally the coefficients, , of a unit matrix are one if and zero otherwise.

The transpose of a matrix , , is what you get if you switch the two indices. Graphically, it turns its rows into its columns and vice versa. The Hermitian “adjoint” is what you get if you switch the two indices and then take the complex conjugate of every element. If you want to take a matrix to the other side of an inner product, you will need to change it to its Hermitian adjoint. “Hermitian matrices”are equal to their Hermitian adjoint, so this does nothing for them.

May indicate:
• Molecular mass. See separate entry.
• Mirror operator.
• Figure of merit.

May indicate:
• Number of rows in a matrix.
• A generic summation index or generic integer.

May indicate:
• Number of columns in a matrix.
• A generic summation index or generic integer.
• A natural number.
and maybe some other stuff.

natural
Natural numbers are the numbers: .

normal
A normal operator or matrix is one that has orthonormal eigenfunctions or eigenvectors. Since eigenvectors are not orthonormal in general, a normal operator or matrix is abnormal! Normal matrices are matrices that commute with their adjoint.

opposite
The opposite of a number is . In other words, it is the additive inverse.

perpendicular bisector
For two given points and , the perpendicular bisector consists of all points that are equally far from as they are from . In two dimensions, the perpendicular bisector is the line that passes through the point exactly half way in between and , and that is orthogonal to the line connecting and . In three dimensions, the perpendicular bisector is the plane that passes through the point exactly half way in between and , and that is orthogonal to the line connecting and . In vector notation, the perpendicular bisector of points and is all points whose radius vector satisfies the equation:

(Note that the halfway point is included in this formula, as is the half way point plus any vector that is normal to .)

phase angle
Any complex number can be written in “polar form” as where both the magnitude and the phase angle are real numbers. Note that when the phase angle varies from zero to , the complex number varies from positive real to positive imaginary to negative real to negative imaginary and back to positive real. When the complex number is plotted in the complex plane, the phase angle is the direction of the number relative to the origin. The phase angle is often called the argument, but so is about everything else in mathematics, so that is not very helpful.

In complex time-dependent waves of the form , and its real equivalent , the phase angle gives the angular argument of the wave at time zero.

May indicate:
• Charge.
• Heat flux density.

May indicate:
• Some function of to be determined.
• Some function of to be determined.

The real part of a complex number. If with and real numbers, then . Note that .

May indicate:
• The radial distance from the chosen origin of the coordinate system.
• often indicates the -th Cartesian component of the radius vector .
• Some ratio.

The position vector. In Cartesian coordinates or . In spherical coordinates . Its three Cartesian components may be indicated by or by or by .

reciprocal
The reciprocal of a number is . In other words, it is the multiplicative inverse.

rot
The rot of a vector is defined as .

scalar
A quantity characterized by a single number.

The sine function, a periodic function oscillating between 1 and -1 as shown in [2, pp. 40-]. Good to remember: .

Stokes' Theorem
This theorem, first derived by Kelvin and first published by someone else I cannot recall, says that for any reasonably smoothly varying vector ,

where the first integral is over any smooth surface area and the second integral is over the edge of that surface. How did Stokes get his name on it? He tortured his students with it, that’s how!

symmetry
Symmetries are operations under which an object does not change. For example, a human face is almost, but not completely, mirror symmetric: it looks almost the same in a mirror as when seen directly. The electrical field of a single point charge is spherically symmetric; it looks the same from whatever angle you look at it, just like a sphere does. A simple smooth glass (like a glass of water) is cylindrically symmetric; it looks the same whatever way you rotate it around its vertical axis.

May indicate:
• Time.

triple product
A product of three vectors. There are two different versions:
• The scalar triple product . In index notation,

where is the index following in the sequence 123123..., and the one preceding it. This triple product equals the determinant formed with the three vectors. Geometrically, it is plus or minus the volume of the parallelepiped that has vectors , , and as edges. Either way, as long as the vectors are normal vectors and not operators,

and you can change the two sides of the dot product without changing the triple product, and/or you can change the sides in the vectorial product with a change of sign.
• The vectorial triple product . In index notation, component number of this triple product is

which may be rewritten as

In particular, as long as the vectors are normal ones,

May indicate:
• The first velocity component in a Cartesian coordinate system.
• An integration variable.

May indicate:
• Volume.

May indicate:
• The second velocity component in a Cartesian coordinate system.
• Magnitude of a velocity (speed).

May indicate:
• Velocity vector.
• Generic vector.

vector
A quantity characterized by a list of numbers. A vector in index notation is a set of numbers indexed by an index . In normal three-dimensional Cartesian space, takes the values 1, 2, and 3, making the vector a list of three numbers, , , and . These numbers are called the three components of .

vectorial product
An vectorial product, or cross product is a product of vectors that produces another vector. If

it means in index notation that the -th component of vector is

where is the index following in the sequence 123123..., and the one preceding it. For example, will equal .

May indicate:
• The third velocity component in a Cartesian coordinate system.
• Weight factor.

Generic vector.

Used in this book to indicate a function of to be determined.

May indicate:
• First coordinate in a Cartesian coordinate system.
• A generic argument of a function.
• An unknown value.

Used in this book to indicate a function of to be determined.

May indicate:
• Second coordinate in a Cartesian coordinate system.
• A second generic argument of a function.
• A second unknown value.

Used in this book to indicate a function of to be determined.

May indicate:
• Third coordinate in a Cartesian coordinate system.
• A third generic argument of a function.
• A third unknown value.