A244057 - OEIS

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A244057

Semiprimes which are concatenation of two consecutive primes.

35, 57, 1317, 1923, 2329, 2931, 4143, 5359, 5961, 6167, 7379, 8997, 103107, 131137, 181191, 193197, 211223, 227229, 281283, 307311, 347349, 367373, 379383, 383389, 421431, 443449, 503509, 547557, 557563, 577587, 587593, 593599, 607613, 619631, 641643, 691701, 709719

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OFFSET

1,1

COMMENTS

The semiprimes in A045533.

LINKS

K. D. Bajpai, Table of n, a(n) for n = 1..10000

Index entries for sequences related to decimal expansion of n

Index entries for sequences related to semiprimes

EXAMPLE

35 is in the sequence because the concatenation of [3, 5] = 35 = 5 * 7, which is semiprime.

1923 is in the sequence because concatenation of [19, 23] = 1923 = 3 * 641, which is semiprime.

1113 is not in the sequence because, though 1113 is concatenation of two consecutive primes [11, 13], 1113 = 3 * 7 * 53, which is not semiprime.

MAPLE

with(numtheory):with(StringTools):A244057:= proc() local a, b, k; a:=ithprime(n); b:=ithprime(n+1); k:=parse(cat(a, b)); if bigomega(k)=2 then RETURN (k); fi; end: seq(A244057 (), n=1..200);

MATHEMATICA

A244057 = {}; Do[t = FromDigits[Flatten[IntegerDigits /@ {Prime[n], Prime[n + 1]}]]; If [PrimeOmega[t] == 2, AppendTo[A244057, t]], {n, 100}]; A244057

CROSSREFS

Cf. A000040, A001358, A030469, A045533, A244007.

Sequence in context: A267738 A219806 A095958 * A219457 A343099 A026064

Adjacent sequences: A244054 A244055 A244056 * A244058 A244059 A244060

KEYWORD

nonn,base

AUTHOR

K. D. Bajpai, Jun 18 2014

STATUS

approved