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A242102 Semiprimes that are the concatenation of a prime and the previous prime. 1
1713, 2319, 2923, 4341, 6159, 7167, 8983, 103101, 151149, 157151, 163157, 167163, 173167, 191181, 197193, 233229, 257251, 277271, 283281, 311307, 337331, 367359, 373367, 421419, 431421, 439433, 449443, 463461, 467463, 479467, 487479, 509503, 521509, 547541, 557547 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

K. D. Bajpai, Table of n, a(n) for n = 1..11480

EXAMPLE

13 and 17 are consecutive primes. Their reverse concatenation = 1713 = 3 * 571, which is semiprime.

19 and 23 are consecutive primes. Their reverse concatenation = 2319 = 3 * 773, which is semiprime.

MAPLE

select(k -> numtheory:-bigomega(k)=2, [seq(parse(cat(ithprime(n+1), ithprime(n))), n=1..200)]);

MATHEMATICA

A242102 = {}; Do[t = FromDigits[Flatten[IntegerDigits /@ {Prime[n], Prime[n - 1]}]]; If[PrimeOmega[t] == 2, AppendTo[A242102, t]], {n, 2, 200}]; A242102

PROG

(PARI)

forprime(p=1, 10^3, q=concat(Str(p), Str(precprime(p-1))); if(bigomega(eval(q))==2, print1(eval(q), ", "))) \\ Derek Orr, Aug 15 2014

CROSSREFS

Cf. A000040, A001358, A007796, A088784.

Sequence in context: A293480 A227218 A280113 * A221481 A008744 A184090

Adjacent sequences:  A242099 A242100 A242101 * A242103 A242104 A242105

KEYWORD

nonn,base

AUTHOR

K. D. Bajpai, Aug 15 2014

STATUS

approved

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Last modified December 10 00:54 EST 2019. Contains 329885 sequences. (Running on oeis4.)