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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<i|&+[d[2]: d in Factorization(i)] eq 2>;

[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.)