21461

Appears in sequences

• From the graph reconstruction problem.at n=7A006655
• Numbers k such that sigma(k+2) = sigma(k).at n=28A007373
• Pell equation solutions (6*a(n))^2 - 37*b(n)^2 = -1 with b(n):=A097730(n), n >= 0.at n=2A097729
• Structured disdyakis triacontahedral numbers (vertex structure 11).at n=10A100158
• Expansion of (1+3*x+9*x^2+9*x^3+9*x^4+3*x^5+x^6) /( (1+x)^2 * (1-x)^5 ).at n=15A175898
• a(n) = (35*n^4 - 35*n^3 + 55*n^2 - 25*n + 6)/6.at n=7A181343
• Half-convolution of sequence A000032 (Lucas) with itself.at n=16A201207
• Bisection of A201207 (half-convolution of the Lucas sequence A000032 with itself); even part.at n=8A203570
• Expansion of x^4/[(1+x)*Product_{k=1..3} (1-k*x)].at n=8A243869
• a(n) = the number of cubes (of integers > 0) that have bit length n.at n=49A365932
• Lesser of 2 successive squarefree semiprimes (k, k+5) sandwiching 4 consecutive nonsquarefree numbers.at n=4A368589
• Numbers k such that the sum of the numbers from 1 to k and that from 1 to k+1 share the same sum of divisors.at n=16A375819
• Numbers k such that s(k) = s(k+2), where s(k) is the sum of odd divisors of k (A000593).at n=8A387920