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A154965 Composites n such that concatenation of n and n-th prime is prime. 1
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified April 19 08:43 EDT 2019. Contains 322241 sequences. (Running on oeis4.)