21901

Appears in sequences

• Strong pseudoprimes to base 27.at n=23A020253
• Numbers k such that the period of the continued fraction for sqrt(k) contains exactly 80 ones.at n=24A031848
• Composite numbers whose prime factors contain no digits other than 1 and 8.at n=8A036308
• Number of distinct differences between consecutive divisors of n! (ordered by size).at n=20A060737
• Pentagonal numbers (A000326) that are the product of 2 palindromes greater than 1.at n=16A115745
• Pentagonal numbers (A000326) which are the sum of 2 other positive pentagonal numbers.at n=27A136117
• a(1)=3; for n > 1, a(n) = 1 + a(n-1) + gcd( a(n-1)*(a(n-1)+2), A073829(a(n-1)) ).at n=32A167053
• Number of (n+1) X (n+1) -5..5 symmetric matrices with every 2 X 2 subblock having sum zero and one or two distinct values.at n=17A211329
• G.f. satisfies: A(x) = exp( Sum_{n>=1} x^n/n * Product_{k>=1} (1 + x^(n*k)*A(x^k)^n) ).at n=13A218552
• Number of third differences of arrays of length 5 of numbers in 0..n.at n=24A228261
• a(n) = n^2*(3*n^2 - 1)/2.at n=11A260810
• Irregular table whose rows list the nontrivial cycles of the ghost iteration A329201, starting with the smallest member.at n=10A329342
• Number of primes of the form k*2^n + 1 with k < 2^n.at n=17A331540
• E.g.f.: Sum_{n>=0} (3^n*x + LambertW(x))^n / n!.at n=3A386656