A154966 Primes p such that the concatenation of p and prime(p) is composite.

3, 5, 7, 11, 13, 19, 29, 31, 37, 43, 47, 53, 59, 67, 73, 79, 89, 97, 101, 103, 107, 109, 113, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317

Offset: 1

Author

Juri-Stepan Gerasimov, Jan 18 2009

Keywords

• nonn
• base
• easy
• less

Examples

			The concatenation of prime 3 and the third prime, 5, is the composite 35, hence 3 is in the sequence.
The concatenation of prime 29 and the 29th prime, 109, is the composite 29109 = 3*31*313, hence 29 is in the sequence.
		

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000040 (primes), A002808 (composites), A045532.

Programs

Magma

[ p: p in PrimesUpTo(320) | not IsPrime(StringToInteger(IntegerToString(p) cat IntegerToString(NthPrime(p)))) ];

Mathematica

Select[Prime[Range[80]],CompositeQ[FromDigits[Flatten[IntegerDigits[ {#,Prime[ #]}]]]]&] (* Harvey P. Dale, Jan 07 2016 *)

Extensions

Edited and extended beyond a(6) by Klaus Brockhaus, Jan 20 2009