A066737 - OEIS

%I #11 Jan 22 2020 16:25:59

%S 22,25,27,32,33,35,52,55,57,72,75,77,112,115,117,132,133,135,172,175,

%T 177,192,195,213,217,219,222,225,231,232,235,237,243,247,252,253,255,

%U 259,261,267,272,273,275,279,289,292,295,297,312,315,319,322,323,325

%N Composite numbers that are concatenations of primes.

%H Robert Israel, <a href="/A066737/b066737.txt">Table of n, a(n) for n = 1..10000</a>

%F A066737 = A152242 \ A000040 = A152242 intersect A002808. - _M. F. Hasler_, Oct 16 2009

%e 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 ccat:= proc(m,n) 10^(1+ilog10(n))*m+n end proc:

%p C[1]:= {2,3,5,7}: P[1]:=C[1]:

%p for n from 2 to 3 do

%p   P[n]:= select(isprime, {seq(i,i=10^(n-1)+1..10^n-1,2)});

%p   C[n]:= P[n];

%p   for m from 1 to n-1 do

%p     C[n]:= C[n] union {seq(seq(ccat(p,q),p =P[m]),q=C[n-m])};

%p   od

%p od:

%p seq(op(sort(convert(remove(isprime,C[n]),list))),n=1..3); # _Robert Israel_, Jan 22 2020

%o (PARI) for(n=1,999, !isprime(n) && is_A152242(n) && print1(n", ")) \\ _M. F. Hasler_, Oct 16 2009

%Y Cf. A121609.

%K base,easy,nonn

%O 1,1

%A _Joseph L. Pe_, Jan 15 2002

%E More terms from Larry Reeves (larryr(AT)acm.org), Apr 03 2002

%E Missing terms added by _M. F. Hasler_, Oct 16 2009