21729

Appears in sequences

• Numbers k such that the continued fraction for sqrt(k) has even period and if the last term of the periodic part is deleted the central term is 98.at n=34A031596
• Numbers k such that the period of the continued fraction for sqrt(k) contains exactly 78 ones.at n=32A031846
• a(n) = number of primes of the form x^2 + 1 <= 2^n.at n=36A083847
• Exponential generating function (1-x^2)^(-1/x).at n=8A121452
• Concatenation of first n taxicab numbers A011541.at n=1A132942
• Numbers k such that k and k^2 use only the digits 1, 2, 4, 7 and 9.at n=16A136998
• Number of primes of the form x^2 + 1 < 2^n.at n=36A174246
• a(1)=1. a(n+1) = Sum_{k=1..n} a(b(k,n)), where b(k,n) is the largest positive integer that, when written in binary, occurs as a substring in both binary k and binary n.at n=45A175491
• Number of nondecreasing arrangements of n+2 numbers in 0..8 with the last equal to 8 and each after the second equal to the sum of one or two of the preceding three.at n=37A190040
• Odd numbers k such that phi(k) and cototient(k) have the same prime signature.at n=29A280927