21771

Appears in sequences

• a(1) = 3; a(n) = smallest number such that the forward as well as the reverse n-th partial concatenation is a prime for n>1. (Reverse concatenation is taken term-wise and not digit-wise).at n=31A083993
• Triangle read by rows: coefficient of x^n in the power series of x/(1 - m*x - x^2 + x^3 - x^5) in row n, column m=1..n+2.at n=48A117744
• Number of planar n X n X n binary triangular grids with mirror symmetry about one altitude with no more than 2 ones in any 5 X 5 X 5 subtriangle.at n=12A153918
• a(n) = 100*n^2 - 49*n + 6.at n=14A157651
• Numbers n that divide the sum of digits of 36^n.at n=43A220364
• Zerofree numbers k such that the product (m+n)*p, where m,n are the first and the last digits of k, and p is the number which is the part of k between m and n, is a divisor of k.at n=27A320292
• a(n) = 3*a(n-1) + 4*a(n-2) + a(n-3), a(0) = 1, a(1) = 1, a(2) = 5.at n=8A321703
• Number of free holey polyiamonds of n cells with simply-connected interiors.at n=7A359524