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

1,1

Links

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

Example

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.

Mathematica

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

Prog

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

Crossrefs

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

Sequence in context: A055072 A059334 A130761 * A072667 A092729 A290817

Adjacent sequences: A154963 A154964 A154965 * A154967 A154968 A154969

Keyword

nonn,base,easy,less

Author

Juri-Stepan Gerasimov, Jan 18 2009

Extensions

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

Status

approved