181776

Appears in sequences

• Number of 2 X 2 matrices with entries mod n and nonzero determinant.at n=20A005353
• Numbers n such that phi(sigma(n)) = 5*phi(n).at n=27A067708
• a(n) = Sum_{k=0..n} n^k * binomial(n,k) * Catalan(k).at n=5A338979