A244057 Semiprimes which are concatenation of two consecutive primes.

35, 57, 1317, 1923, 2329, 2931, 4143, 5359, 5961, 6167, 7379, 8997, 103107, 131137, 181191, 193197, 211223, 227229, 281283, 307311, 347349, 367373, 379383, 383389, 421431, 443449, 503509, 547557, 557563, 577587, 587593, 593599, 607613, 619631, 641643, 691701, 709719

Offset: 1

Author

K. D. Bajpai, Jun 18 2014

Keywords

• nonn
• base

Comments

The semiprimes in A045533.

Examples

			35 is in the sequence because the concatenation of [3, 5] = 35 = 5 * 7, which is semiprime.
1923 is in the sequence because concatenation of [19, 23] = 1923 = 3 * 641, which is semiprime.
1113 is not in the sequence because, though 1113 is concatenation of two consecutive primes [11, 13], 1113 = 3 * 7 * 53, which is not semiprime.
		

Links

• K. D. Bajpai, Table of n, a(n) for n = 1..10000
• Index entries for sequences related to decimal expansion of n
• Index entries for sequences related to semiprimes

Crossrefs

Cf. A000040, A001358, A030469, A045533, A244007.

Programs

Maple

with(numtheory):with(StringTools):A244057:= proc() local a,b,k; a:=ithprime(n); b:=ithprime(n+1); k:=parse(cat(a,b)); if bigomega(k)=2 then RETURN (k); fi; end: seq(A244057 (), n=1..200);

Mathematica

A244057 = {}; Do[t = FromDigits[Flatten[IntegerDigits /@ {Prime[n], Prime[n + 1]}]]; If [PrimeOmega[t] == 2, AppendTo[A244057, t]], {n, 100}]; A244057