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A196104 Semiprimes  having  identical digits. 1
4, 6, 9, 22, 33, 55, 77, 111, 1111, 11111, 1111111, 11111111111, 11111111111111111, 2222222222222222222, 3333333333333333333, 5555555555555555555, 7777777777777777777, 22222222222222222222222, 33333333333333333333333, 55555555555555555555555 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..20.

FORMULA

{4, 6, 9} union A131317. - R. J. Mathar, Oct 27 2011

EXAMPLE

a(12) = 11111111111 = 21649 * 513239 is semiprime.

MAPLE

with(numtheory):for n from 1 to 23 do:for b from 1 to 9 do:x:=(((10^n)- 1)/9)*b:if bigomega(x)=2 then printf(`%d, `, x):else fi:od:od:

MATHEMATICA

Select[FromDigits/@Flatten[Table[PadRight[{}, i, n], {i, 25}, {n, 9}], 1], PrimeOmega[ #] ==2&] (* Harvey P. Dale, Mar 11 2019 *)

PROG

(PARI) print1("4, 6, 9"); for(n=1, 20, t=10^n\9; if(bigomega(t)==2, print1(", "t)); if(isprime(t), forprime(p=2, 7, print1(", "p*t)))) \\ Charles R Greathouse IV, Oct 27 2011

CROSSREFS

Cf. A001358, A046328.

Sequence in context: A118690 A084994 A046328 * A046376 A229129 A272436

Adjacent sequences:  A196101 A196102 A196103 * A196105 A196106 A196107

KEYWORD

nonn,base

AUTHOR

Michel Lagneau, Oct 27 2011

STATUS

approved

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Last modified April 25 16:08 EDT 2019. Contains 322461 sequences. (Running on oeis4.)