Clue  Letter 
change in a quantity, usually as it relates to change in another quantity  
extension of the factorial function to complex numbers  
in number theory, the sum of the divisors of a positive integer  
representation of a group, or radius in spherical polar coordinates  
commonly used to denote a plane angle in Euclidean geometry  
character in representation theory, or chromatic number in graph theory  
commonly used to denote complex numbers, especially roots of unity  
Lagrange multipliers in calculus, or eigenvalues in linear algebra  
a summation, has a subscript and sometimes a superscript  
the connectivity of a graph, or the local connectivity of two vertices  
the limit of the ratios of successive terms in the Fibonacci sequence  
 Clue  Letter 
an arbitrarily small positive quantity, used by Erdős to refer to children  
the Möbius function, which always takes on a value of 1, 0, or 1  
the polygamma function, or the reciprocal Fibonacci constant  
number of positive integers less than and coprime to a given integer  
the independent variable for the Fourier transform of a function  
in graph theory, the smallest degree of any vertex in a graph  
coefficient of the real component of a complex number  
coefficient of the imaginary component of a complex number  
twice the arcsine of 1, or the number of primes less than a given number  
the least ordinal which the reals cannot map onto surjectively  
the function whose zeroes are the subject of the Reimann hypothesis  
probably the most underused Greek letter, not sure why it isn't more common  
looks too much like a 1 to be used  
looks too much like a 0 to be used  
looks too much like a V to be used  
looks too much like a U to be used  
a bit too long to be used as a mathematical variable, methinks  
