A154965 Composites n such that concatenation of n and n-th prime is prime.

4, 6, 12, 18, 27, 30, 35, 36, 42, 45, 46, 54, 56, 57, 58, 82, 85, 92, 93, 99, 115, 117, 119, 121, 123, 136, 141, 145, 147, 159, 161, 171, 175, 176, 177, 206, 215, 222, 225, 237, 243, 261, 273, 274, 282, 285, 291, 295, 296, 303, 309, 315, 319, 322, 327, 328, 332

Offset

1,1

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Example

Concatenation of composite 4 and fourth prime 7 is the prime 47, hence 4 is in the sequence. Concatenation of composite 36 and 36th prime 151 is the prime 36151, hence 36 is in the sequence.

Mathematica

concatQ[n_]:=PrimeQ[FromDigits[Join[IntegerDigits[n], IntegerDigits[ Prime[n]]]]]; nn=400; Select[Complement[Range[nn], Prime[Range[ PrimePi[nn]]]], concatQ] (* Harvey P. Dale, Jun 05 2011 *)

Prog

(Magma) [ n: n in [1..340] | not IsPrime(n) and IsPrime(StringToInteger(IntegerToString(n) cat IntegerToString(NthPrime(n)))) ];

Crossrefs

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

Sequence in context: A310600 A296507 A072568 * A249630 A109290 A130441

Adjacent sequences: A154962 A154963 A154964 * A154966 A154967 A154968

Keyword

nonn,base,easy,less

Author

Juri-Stepan Gerasimov, Jan 18 2009

Extensions

Edited and extended beyond a(4) by Klaus Brockhaus, Jan 20 2009

Status

approved