A175355 Noncomposite reverse concatenations of divisors of n, sorted by n.

1, 31, 421, 71, 131, 191, 2551, 311, 391331, 431, 48241612864321, 911371, 971, 1031, 1091, 1173913931, 1511, 1571, 1811, 1931, 2111, 2411, 24412261421, 24719131, 2711, 289171, 29214673421, 3014371, 30910331, 32565251351, 3371, 3491, 3671

Offset: 1

Author

Jaroslav Krizek, Apr 20 2010

Keywords

• nonn
• base

Examples

			For n = 9; a(9) = 391331 because A089374(9) = 39 and divisors of 39 are 1, 3, 13, 39; reverse concatenation of divisors A176558(21) = 391331 is noncomposite.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Subsequence of A176558(n). Supersequence of A055781.

Programs

Maple

rcd:= proc(n) local D,T,i;
  D:= sort(convert(numtheory:-divisors(n),list));
  T:= D[1];
  for i from 2 to nops(D) do
    T:= T + 10^(1+ilog10(T))*D[i]
  od;
  T
end proc:
select(t -> t=1 or isprime(t), map(rcd, [$1..1000])); # Robert Israel, Aug 12 2020

Formula

a(n) = A176558(A089374(n)).

a(n) = A069582(n-1), n>1. [R. J. Mathar, May 03 2010]

Extensions

Edited by Charles R Greathouse IV, Apr 30 2010