20827

Appears in sequences

• a(0) = 1, a(n) = 17*n^2 + 2 for n>0.at n=35A010007
• Number of plane regions after drawing (in general position) a convex n-gon and all its diagonals.at n=26A027927
• Numbers n such that Fibonacci(n) is not squarefree, but for all proper divisors k of n, Fibonacci(k) is squarefree.at n=39A065069
• Number of permutations satisfying -k<=p(i)-i<=r and p(i)-i not in I, i=1..n, with k=2, r=3, I={-1}.at n=15A080011
• Pentagonal numbers (A000326) which are the sum of 2 other positive pentagonal numbers.at n=26A136117
• Smallest number m such that prime(n) is a factor of both m and sigma(m).at n=16A156099
• A (-1,1) Somos-4 sequence associated with the elliptic curve y^2 + y = x^3 + x.at n=12A178384
• G.f.: Sum_{n>=0} x^n * (1+x)^sigma(n).at n=15A227236
• a(n) = n*prime(prime(n)) - prime(n)^2.at n=51A230098
• Semiprimes of the form n*(3*n-1)/2.at n=19A245365
• Number of set partitions of [n] such that j is member of block b only if b = 1 or at least one of j-1, ..., j-4 is member of a block >= b-1.at n=9A287667
• Semiprimes of the form p(6p - 1).at n=11A330409