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 A175354 Numbers m such that reverse concatenations of divisors of m are nonprimes. 4

%I

%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

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Last modified April 16 10:22 EDT 2021. Contains 343036 sequences. (Running on oeis4.)