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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
OFFSET
1,1
LINKS
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
KEYWORD
nonn,base,easy,less
AUTHOR
EXTENSIONS
Edited and extended beyond a(4) by Klaus Brockhaus, Jan 20 2009
STATUS
approved