21687

Appears in sequences

• Indices of primes in sequence defined by A(0) = 67, A(n) = 10*A(n-1) - 23 for n > 0.at n=19A101529
• Number of n X n binary arrays symmetric under horizontal and vertical reflection with all ones connected only in a 00100-11111-00100 pattern in any orientation.at n=16A147005
• Number of -1..1 arrays x(0..n-1) of n elements with zero sum and no two consecutive declines, no adjacent equal elements, and no element more than one greater than the previous (random base sawtooth pattern).at n=28A200174
• Take a squarefree semiprime and take the difference of its prime factors. If it is a squarefree semiprime repeat the process. Sequence lists the squarefree semiprimes that generate other squarefree semiprimes only in the first k steps of this process. Case k = 4.at n=40A296811
• Number of partitions of n with up to ten distinct kinds of 1.at n=17A320697