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