A175354 - OEIS

A175354

Numbers m such that reverse concatenations of divisors of m are nonprimes.

1, 2, 5, 6, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 20, 21, 22, 23, 24, 26, 27, 28, 29, 30, 32, 33, 34, 35, 36, 37, 38, 40, 41, 42, 44, 45, 46, 47, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81

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OFFSET

1,2

COMMENTS

See A176558(n) = reverse concatenation of divisors of n. See A176588 for corresponding values of reverse concatenations. Complement of A089374(n) for n >= 2.

LINKS

Table of n, a(n) for n=1..71.

EXAMPLE

Divisors of 12: 1, 2, 3, 4, 6, 12; reverse concatenation of divisors 1264321 is nonprime number.

MAPLE

filter:= proc(n) local r, L, i;

L:= sort(convert(numtheory:-divisors(n), list));

r:= L[1];

for i from 2 to nops(L) do

r:= r + 10^(1+ilog10(r))*L[i]

od;

not isprime(r)