21421

Appears in sequences

• Expansion of 1/((1-2*x)*(1-6*x)*(1-9*x)).at n=4A016306
• a(n) = s(1)t(n) + s(2)t(n-1) + ... + s(k)t(n+1-k), where k = [ (n+1)/2 ], s = (1, p(1), p(2), ...), t = (F(2), F(3), ...).at n=16A024481
• Numerators of continued fraction convergents to sqrt(526).at n=6A042006
• Numbers that are both centered triangular numbers (A005448) and centered hexagonal numbers (A003215).at n=3A107118
• Semiprimes in A003215.at n=36A113530
• Number of walks within N^3 (the first octant of Z^3) starting at (0,0,0) and consisting of n steps taken from {(-1, -1, 1), (0, 1, -1), (0, 1, 0), (1, 0, 0), (1, 1, 1)}.at n=7A151157
• a(n) = 1 + n*(n+1)*(n-1)/2.at n=35A158842
• a(n) = numerator((Zeta(2, 1/3) - Zeta(2, n + 1/3))/9), where Zeta(n, z) is the Hurwitz Zeta function.at n=4A173983
• Semiprime centered triangular numbers.at n=43A184481
• Centered 36-gonal numbers.at n=34A195316
• Numbers k such that w(k), w(k+1), and w(k+2) are all odd, where w is A360519.at n=5A361106
• Constant term in the expansion of (1 + x^2 + y^2 + 1/(x*y))^n.at n=10A361657
• Number of compositions of 6*n-4 into parts 5 and 6.at n=15A373964
• Squarefree semiprimes that are centered triangular numbers.at n=40A380913
• a(n) = Sum_{k=0..floor(2*n/5)} binomial(k,2*n-5*k).at n=46A391265
• a(n) = Sum_{k=0..floor(3*n/5)} binomial(k+1,3*n-5*k).at n=29A391843