%I #10 Apr 21 2020 09:14:16
%S 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,
%T 33,34,35,36,37,38,40,41,42,44,45,46,47,49,50,51,52,53,54,55,56,57,58,
%U 59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81
%N Numbers m such that reverse concatenations of divisors of m are nonprimes.
%C See A176558(n) = reverse concatenation of divisors of n. See A176588 for corresponding values of reverse concatenations. Complement of A089374(n) for n >= 2.
%e Divisors of 12: 1, 2, 3, 4, 6, 12; reverse concatenation of divisors 1264321 is nonprime number.
%p filter:= proc(n) local r,L,i;
%p L:= sort(convert(numtheory:-divisors(n),list));
%p r:= L[1];
%p for i from 2 to nops(L) do
%p r:= r + 10^(1+ilog10(r))*L[i]
%p od;
%p not isprime(r)
%p end proc:
%p select(filter, [$1..100]); # _Robert Israel_, Apr 21 2020
%o (PARI) rcd(n)=my(d=divisors(n));n=1;for(i=2,#d,n=glue(d[i],n));n
%o glue(a,b)=a*10^#Str(b)+b
%o for(m=1,81,if(!isprime(rcd(m)),print1(m", ")))
%Y Cf. A089374, A176558, A176588.
%K nonn,base
%O 1,2
%A _Jaroslav Krizek_, Apr 20 2010
%E Program, editing, and extension by _Charles R Greathouse IV_, Apr 23 2010
%E Corrected by _Jaroslav Krizek_, Apr 26 2010