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A105991 Semiprimes consisting of digits 0 and 1 only. 1

%I

%S 10,111,1011,1101,1111,10001,11101,11111,100001,101011,110101,110111,

%T 111001,111011,111101,1000001,1000011,1000101,1000111,1001001,1001101,

%U 1001111,1010001,1010111,1100111,1101011,1110001,1110011,1110101,1111001,1111111,10000001

%N Semiprimes consisting of digits 0 and 1 only.

%C This includes repunit semiprimes also.

%C Intersection of A001358 (semiprimes) and A007088 (numbers written in base 2). - _Michel Marcus_, Jan 27 2015

%H Zak Seidov, <a href="/A105991/b105991.txt">Table of n, a(n) for n = 1..531</a>

%e a(3)=1011 is a term because 1011 is a semiprime and contain digits 0 and 1 only.

%p N:= 12: # to get all terms with at most N digits

%p cands[1]:= {1}:

%p for d from 2 to N do

%p cands[d]:= map(t -> (10*t, 10*t+1), cands[d-1])

%p od:

%p select(numtheory:-bigomega = 2, `union`(seq(cands[d],d=1..N)));

%p # if using Maple 11 or lower, uncomment the next line

%p # sort(convert(%,list)); # _Robert Israel_, Jan 27 2015

%o (PARI) isok(n) = (bigomega(n)==2) && (vecmax(digits(n)) <= 1); \\ _Michel Marcus_, Jan 27 2015

%Y Cf. A001358, A007088.

%K base,nonn

%O 1,1

%A _Shyam Sunder Gupta_, Apr 29 2005

%E Data corrected by _Zak Seidov_, Jan 27 2015

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Last modified September 17 04:49 EDT 2019. Contains 327119 sequences. (Running on oeis4.)