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A066737 Composite numbers that are concatenations of primes.

%I A066737 #11 Jan 22 2020 16:25:59
%S A066737 22,25,27,32,33,35,52,55,57,72,75,77,112,115,117,132,133,135,172,175,
%T A066737 177,192,195,213,217,219,222,225,231,232,235,237,243,247,252,253,255,
%U A066737 259,261,267,272,273,275,279,289,292,295,297,312,315,319,322,323,325
%N A066737 Composite numbers that are concatenations of primes.
%H A066737 Robert Israel, <a href="/A066737/b066737.txt">Table of n, a(n) for n = 1..10000</a>
%F A066737 A066737 = A152242 \ A000040 = A152242 intersect A002808. - _M. F. Hasler_, Oct 16 2009
%e A066737 72 is the concatenation of primes 7 and 2. 132 is the concatenation of primes 13 and 2. 225 is the concatenation of 2, 2 and 5.
%p A066737 ccat:= proc(m,n) 10^(1+ilog10(n))*m+n end proc:
%p A066737 C[1]:= {2,3,5,7}: P[1]:=C[1]:
%p A066737 for n from 2 to 3 do
%p A066737   P[n]:= select(isprime, {seq(i,i=10^(n-1)+1..10^n-1,2)});
%p A066737   C[n]:= P[n];
%p A066737   for m from 1 to n-1 do
%p A066737     C[n]:= C[n] union {seq(seq(ccat(p,q),p =P[m]),q=C[n-m])};
%p A066737   od
%p A066737 od:
%p A066737 seq(op(sort(convert(remove(isprime,C[n]),list))),n=1..3); # _Robert Israel_, Jan 22 2020
%o A066737 (PARI) for(n=1,999, !isprime(n) && is_A152242(n) && print1(n", ")) \\ _M. F. Hasler_, Oct 16 2009
%Y A066737 Cf. A121609.
%K A066737 base,easy,nonn
%O A066737 1,1
%A A066737 _Joseph L. Pe_, Jan 15 2002
%E A066737 More terms from Larry Reeves (larryr(AT)acm.org), Apr 03 2002
%E A066737 Missing terms added by _M. F. Hasler_, Oct 16 2009