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 A216378 n such that n*10^n + 1 is semiprime. 1
 2, 5, 13, 28, 34, 36, 39 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This is to A007647 as semiprimes A001358 is to primes A000040. The corresponding semiprimes are A216376 = {201, 500001, 130000000000001, 280000000000000000000000000001, ...}. LINKS FORMULA {n such that A216376(n) is in A001358}. EXAMPLE a(1) = 2 because 2 * 10^2 + 1 = 201 = 3 * 67. a(2) = 5 because  5 * 10^5 + 1 = 500001 = 3 * 166667. a(3) = 13 because 13*10^13 + 1 = 130000000000001 = 6529 * 19911165569. a(4) = 28 because 28 * 10^28 + 1 = 29 * 9655172413793103448275862069. MATHEMATICA Select[Range[40], PrimeOmega[# 10^# + 1] == 2 &] (* Alonso del Arte, Sep 08 2012 *) PROG (MAGMA) IsSemiprime:=func; [n: n in [1..70] | IsSemiprime(s) where s is n*10^n+1]; // Vincenzo Librandi, May 10 2014 CROSSREFS Cf. A001358, A007647, A064748, A216376. Cf. similar sequences listed in A242203. Sequence in context: A126656 A272794 A026522 * A225690 A193044 A122491 Adjacent sequences:  A216375 A216376 A216377 * A216379 A216380 A216381 KEYWORD nonn,more AUTHOR Jonathan Vos Post, Sep 06 2012 STATUS approved

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Last modified June 25 12:01 EDT 2019. Contains 324352 sequences. (Running on oeis4.)