20961

Appears in sequences

• a(n) = (n^4 + n^2 + 2*n)/4.at n=17A006528
• Divisors of 10^16 - 1.at n=43A111211
• Numbers a = b + c where a, b, and c contain the same decimal digits.at n=27A203024
• Pseudoprimes to base 10 that are not squarefree.at n=8A306449
• Expansion of e.g.f. exp(x/(1 - 2*x)^2).at n=5A380639
• Irregular triangle read by rows: T(n,k) is the number of polyominoes of size k, i.e., connected subsets of k square cells (or vertices), of the n X n flat torus, up to cyclic shifts and reflections of rows and columns; 1 <= k <= n^2.at n=42A385383