Clue | Letter |

character in representation theory, or chromatic number in graph theory | |

twice the arcsine of 1, or the number of primes less than a given number | |

a summation, has a subscript and sometimes a superscript | |

in number theory, the sum of the divisors of a positive integer | |

Lagrange multipliers in calculus, or eigenvalues in linear algebra | |

commonly used to denote a plane angle in Euclidean geometry | |

the independent variable for the Fourier transform of a function | |

commonly used to denote complex numbers, especially roots of unity | |

an arbitrarily small positive quantity, used by Erdős to refer to children | |

in graph theory, the smallest degree of any vertex in a graph | |

representation of a group, or radius in spherical polar coordinates | |

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 | |