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 A210711 Semiprimes formed by concatenating n, n, and 1 for n = 1, 2, 3,.... 0
 111, 221, 551, 771, 11111, 14141, 15151, 16161, 19191, 23231, 24241, 29291, 30301, 34341, 36361, 37371, 38381, 39391, 42421, 44441, 47471, 50501, 53531, 55551, 56561, 59591, 62621, 68681, 70701, 74741, 75751, 77771, 79791, 81811, 83831, 84841, 87871, 91911, 95951, 96961, 1001001 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This is to A210511 as semiprimes A001358 are to primes A000040. LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE a(1) = 11 because 111 = 3 * 37. a(2) = 221 because 221 = 13 * 17. 331 is not in the sequence, because it is a prime. a(5) = 11111 because "11" concatenated with "11" concatenated with "1" = 11111 = 41 * 271. MAPLE read("transforms"): for n from 1 to 100 do     L := [n, n, 1] ;     p := digcatL(L) ;     if numtheory[bigomega](p) = 2 then         print(p) ;     end if; end do: # R. J. Mathar, Jan 30 2013 PROG (MAGMA) IsSemiprime:=func; [nn1: n in [1..100] | IsSemiprime(nn1) where nn1 is Seqint([1] cat Intseq(n) cat Intseq(n))]; // Bruno Berselli, Jan 30 2013 CROSSREFS Cf. A001358, A210511. Sequence in context: A082945 A316849 A316850 * A084043 A291266 A033284 Adjacent sequences:  A210708 A210709 A210710 * A210712 A210713 A210714 KEYWORD nonn,base,easy,changed AUTHOR Jonathan Vos Post, Jan 29 2013 STATUS approved

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Last modified July 23 16:10 EDT 2019. Contains 325258 sequences. (Running on oeis4.)