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

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)
end proc:
select(filter, [$1..100]); # Robert Israel, Apr 21 2020

Prog

(PARI) rcd(n)=my(d=divisors(n)); n=1; for(i=2, #d, n=glue(d[i], n)); n
glue(a, b)=a*10^#Str(b)+b
for(m=1, 81, if(!isprime(rcd(m)), print1(m", ")))

Crossrefs

Cf. A089374, A176558, A176588.

Sequence in context: A352325 A228089 A126971 * A224778 A286727 A090946

Adjacent sequences: A175351 A175352 A175353 * A175355 A175356 A175357

Keyword

nonn,base

Author

Jaroslav Krizek, Apr 20 2010

Extensions

Program, editing, and extension by Charles R Greathouse IV, Apr 23 2010

Corrected by Jaroslav Krizek, Apr 26 2010

Status

approved