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A261364 Semiprimes that are the concatenation of n 1's, 2^n and n 1's 2
121, 1118111, 1111161111, 11111164111111, 111111111512111111111, 1111111111114096111111111111, 111111111111181921111111111111, 111111111111111638411111111111111, 111111111111111111262144111111111111111111, 11111111111111111111104857611111111111111111111 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Inspiration was symmetry and visual simplicity.

LINKS

Table of n, a(n) for n=1..10.

EXAMPLE

a(1) = 121 because the concatenation of 1, 2 and 1 is a semiprime number.

a(2) = 1118111 because the concatenation of 111, 8 and 111 is a semiprime number.

a(3) = 1111161111 because the concatenation of 1111, 16 and 1111 is a semiprime number.

MAPLE

ncat:= (a, b) -> 10^(1+ilog10(b))*a+b:

f:= proc(n) local N;

  N:= ncat(ncat((10^n-1)/9, 2^n), (10^n-1)/9);

  if numtheory:-bigomega(N) = 2 then N else NULL fi

end proc:

seq(f(n), n=1..25); # Robert Israel, Oct 04 2015

PROG

(PARI) for(n=1, 25, if(bigomega(k=eval(Str((10^n - 1)/9, 2^n, (10^n - 1)/9))) == 2, print1(k", ")))

CROSSREFS

Cf. A001358, A002275, A068817, A070220, A070746, A262399.

Sequence in context: A135825 A221392 A227301 * A317956 A136094 A317197

Adjacent sequences:  A261361 A261362 A261363 * A261365 A261366 A261367

KEYWORD

nonn,base

AUTHOR

Altug Alkan, Oct 02 2015

STATUS

approved

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Last modified November 21 22:40 EST 2019. Contains 329383 sequences. (Running on oeis4.)