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

Author

Jaroslav Krizek, Apr 20 2010

Keywords

• nonn
• base

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.

Examples

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

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A089374, A176558, A176588.

Programs

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

Mathematica

Select[Range[90],!PrimeQ[FromDigits[Flatten[IntegerDigits/@Reverse[Divisors[#]]]]]&] (* Harvey P. Dale, Mar 21 2025 *)

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", ")))

Extensions

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

Corrected by Jaroslav Krizek, Apr 26 2010