21105

Appears in sequences

• Odd primitive abundant numbers.at n=28A006038
• [ n(n-1)(n-2)(n-3)/17 ].at n=26A011927
• Expansion of e.g.f. theta_3^(1/2).at n=8A015664
• a(n) is the concatenation of n and 5n.at n=20A019553
• Numbers that, when expressed in base 3 and then interpreted in base 10, yield a multiple of the original number.at n=33A032537
• Number of directed multigraphs with loops on 3 nodes with n arcs.at n=12A050927
• Number of periodic palindromic structures of length n using exactly four different symbols.at n=17A056510
• Odd primitive numbers such that n! divided by product of factorials of all proper divisors of n is not an integer.at n=25A075460
• Beginning with 1, a(i)*a(j) + 2 is prime for all i, j, i != j.at n=6A083519
• Round(1000*x), where x is the solution to x = 5^(n-x).at n=23A104744
• Odd primitive abundant numbers n such that n = x^2 + x + y^2 with y^2 < 2*x; a subsequence of A006038.at n=4A136476
• a(n) = ((2^b-1)/phi(n))*Sum_{d|n} Moebius(n/d)*d^(b-1) for b = 4.at n=36A160892
• Number of arrangements of 4 numbers x(i) in -n..n with the sum of x(i)*x(i+1) equal to zero.at n=22A188359
• Irregular triangle of odd primitive abundant numbers (A006038) in which row n has numbers with n distinct prime factors.at n=30A188439
• Number of undirected circular permutations i_0, i_1, ..., i_n of 0, 1, ..., n such that all the n+1 adjacent distances |i_0-i_1|, |i_1-i_2|, ..., |i_{n-1}-i_n|, |i_n-i_0| are perfect squares.at n=16A229543
• Odd numbers in A192274.at n=34A243104
• Odd numbers k such that the product of factorials of proper divisors of k does not divide k!at n=38A248694
• Irregular triangle read by rows where row n lists all odd primitive abundant numbers with n prime factors, counted with multiplicity.at n=27A287646
• Solution of the complementary equation a(n) = 2*a(n-1) - a(n-3) + b(n-2), where a(0) = 1, a(1) = 3, a(2) = 5, b(0) = 2, b(1) = 4, b(2) = 6, and (a(n)) and (b(n)) are increasing complementary sequences.at n=16A295617
• Expansion of Product_{k>=1} 1/(1+x^k)^((-1)^k*k^2).at n=18A307484