A175355 - OEIS

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

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OFFSET

1,2

LINKS

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

FORMULA

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

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

EXAMPLE

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.

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;

end proc:

select(t -> t=1 or isprime(t), map(rcd, [$1..1000])); # Robert Israel, Aug 12 2020

CROSSREFS

Subsequence of A176558(n). Supersequence of A055781.

Sequence in context: A268137 A089375 A069582 * A068658 A068672 A274202