A238500 - OEIS

%I #6 Mar 03 2014 08:38:55

%S 233347,233911,239929,337397,373613,379397,733331,796337,1321997,

%T 1933331,2333347,2333533,2339929,2392333,2393257,2393761,2939971,

%U 3136373,3165713,3217337,3319733,3499277,3539311,3727397,3733967,3739103,3739199,3739397,3739433

%N Primes which are the concatenation of two primes in exactly four ways.

%H Giovanni Resta, <a href="/A238500/b238500.txt">Table of n, a(n) for n = 1..10000</a>

%e 233347 is in the sequence because 2, 33347, 23, 3347, 233, 347, 2333 and 47 are all primes, so there are four ways.

%t spl[n_] := Block[{d = IntegerDigits@n, c = 0, z}, z = Length@d; Do[If[PrimeQ@ FromDigits@ Take[d, k] && d[[k + 1]] > 0 && PrimeQ@ FromDigits@ Take[d, k - z], c++], {k, z - 1}]; c]; Select[ Prime@ Range@ 250000, spl[#] == 4 &] (* _Giovanni Resta_, Mar 03 2014 *)

%Y Cf. A105184, A238056, A238057, A238499.

%K nonn,base

%O 1,1

%A _Colin Barker_, Feb 27 2014