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%I #18 Sep 06 2014 12:20:10
%S 1713,2319,2923,4341,6159,7167,8983,103101,151149,157151,163157,
%T 167163,173167,191181,197193,233229,257251,277271,283281,311307,
%U 337331,367359,373367,421419,431421,439433,449443,463461,467463,479467,487479,509503,521509,547541,557547
%N Semiprimes that are the concatenation of a prime and the previous prime.
%H K. D. Bajpai, <a href="/A242102/b242102.txt">Table of n, a(n) for n = 1..11480</a>
%e 13 and 17 are consecutive primes. Their reverse concatenation = 1713 = 3 * 571, which is semiprime.
%e 19 and 23 are consecutive primes. Their reverse concatenation = 2319 = 3 * 773, which is semiprime.
%p select(k -> numtheory:-bigomega(k)=2, [seq(parse(cat(ithprime(n+1),ithprime(n))), n=1..200)]);
%t A242102 = {}; Do[t = FromDigits[Flatten[IntegerDigits /@ {Prime[n], Prime[n - 1]}]]; If[PrimeOmega[t] == 2, AppendTo[A242102, t]], {n, 2, 200}]; A242102
%o (PARI)
%o 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
%Y Cf. A000040, A001358, A007796, A088784.
%K nonn,base
%O 1,1
%A _K. D. Bajpai_, Aug 15 2014
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