21097

Appears in sequences

• Strong pseudoprimes to base 38.at n=19A020264
• Odd 10-gonal (or decagonal) numbers.at n=36A028993
• Numbers k such that the continued fraction for sqrt(k) has odd period and if the last term of the periodic part is deleted the two central terms are both 15.at n=15A031603
• a(n) = n^2*(n^2+3)/4.at n=16A039623
• Numbers n such that 57*2^n-1 is prime.at n=29A050554
• Numbers k such that the k-th triangular number contains only digits {2,3,5}.at n=9A119153
• Floor of sum of the first 10^n square roots.at n=3A137262
• Number of n X n binary arrays symmetric under horizontal reflection with all ones connected only in a 10001-11111 pattern in any orientation.at n=10A147084
• a(n) = 73*n^2.at n=17A174334
• The location of records in A210700.at n=28A210701
• Series reversion of the g.f. of A228862.at n=18A228863
• 10-gonal (or decagonal) numbers with prime indices.at n=20A267217
• On a spirally numbered square grid, with labels starting at 1, this is the number of the last cell that an (n,n+1) leaper reaches before getting trapped, or -1 if it never gets trapped.at n=34A343179
• Composite numbers for which A324644(n)/A324198(n) = 2 and sigma(n) == 2 (mod 4).at n=19A371082