20273

Appears in sequences

• Numbers k such that 3*k^2 - 3*k + 1 is both a square (A000290) and a centered hexagonal number (A003215).at n=4A001922
• Geometric mean of phi(n) and sigma(n) is an integer, n odd.at n=37A015705
• Odd k for which k+2^m is composite for all m < k.at n=12A033919
• Expansion of (1-x)^2/((1+x^2)*(1-4*x+x^2)).at n=8A099488
• 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), (-1, 1, 0), (0, 0, -1), (1, 0, 0)}.at n=12A148014
• Numbers n such that sigma(n)/phi(n) = 49/36.at n=2A164650
• Numbers n such that phi(n)=2*phi(n-1).at n=22A171271
• a(n) = n^3/6 + 3*n^2/4 + 7*n/3 + 7/8 + (-1)^n/8.at n=48A173154
• Number of strictly increasing arrangements of 4 nonzero numbers in -(n+2)..(n+2) with sum zero.at n=44A188123
• Second elementary symmetric function of the first n terms of (1,2,3,5,8,...).at n=8A203245
• a(n) = Sum_{k=1..n} binomial(k+2,2) * floor(n/k).at n=43A366984