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An untouchable number is a positive integer that is not the sum of the proper divisors of any number. The first few are 2, 5, 52, 88, 96, 120, 124, 146, ... (OEIS A005114). ...

"Brick" is an informal term for a cuboid.

For a finite group of h elements with an n_ith dimensional ith irreducible representation, sum_(i)n_i^2=h.

The sporadic group HJ, also denoted J_2.

The function lambda(n)=(-1)^(Omega(n)), (1) where Omega(n) is the number of not necessarily distinct prime factors of n, with Omega(1)=0. The values of lambda(n) for n=1, 2, ...

Combine the two above squares on the left into the single large square on the right.

A finite group L is quasisimple if L=[L,L] and L/Z(L) is a simple group.

A property of finite simple groups which is known for all such groups.

B_(p+k)=B_k+B_(k+1) (mod p), when p is prime and B_n is a Bell number.

The generalization of a Voronoi polygon to n dimensions for n>2.