21032

Appears in sequences

• Decimal expansion of a(n) is given by the first n terms of the periodic sequence with initial period 2,1,0,3.at n=4A037736
• Constant term of the reduction by x^2 -> x+1 of the polynomial p(n,x)=(2x+1)(2x+2)...(2x+n).at n=6A192941
• G.f.: (1+x^4)/(1-x-x^8).at n=50A193942
• G.f.: sqrt(1 + 2*Sum_{n>=1} 2^n*x^(n^2)).at n=11A227315
• Numbers k such that if x = sigma(k) + tau(k) - k then k = sigma(x) + tau(x) - x.at n=19A238226
• Number of square permutations of 2n things.at n=4A279200
• Numbers k such that sopfr(k) = tau(k)^2.at n=18A305026
• Lesser members of dihedral amicable pairs: numbers (m, k) such that t(m) = t(k) = m + k, where t(k) = sigma(k) + d(k).at n=5A320457
• a(n) is the number of almost almost unitary polyominoes of size n. An almost almost unitary polyomino is one in which all but 2 of its perimeter walls have length 1.at n=17A362709