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A175354 a(n) = numbers m such that reverse concatenations of divisors of m are nonprimes. 4


%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 a(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.

%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",")))

%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 January 20 19:53 EST 2020. Contains 331096 sequences. (Running on oeis4.)