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A368337 Semiprimes that contain only digits 4 and 9.

%I A368337 #12 Dec 21 2023 18:58:30
%S A368337 4,9,49,94,949,4449,4499,9449,44494,44949,44999,49949,94499,94994,
%T A368337 99449,99494,99949,444494,444949,494449,494999,499949,944494,944949,
%U A368337 944999,949999,994999,999494,4444449,4444499,4449949,4449999,4494449,4494499,4494949,4494999,4499449,4499494,4944449,4944499
%N A368337 Semiprimes that contain only digits 4 and 9.
%C A368337 The only terms that are squares are 4, 9 and 49.
%C A368337 Numbers of n-digit terms for n = 1...20: {2, 2, 1, 3, 13, 11, 31, 39, 78, 159, 383, 541, 1302, 2047, 4268, 6926, 16248, 27172, 57397, 94581}.
%H A368337 Robert Israel, <a href="/A368337/b368337.txt">Table of n, a(n) for n = 1..10000</a>
%e A368337 a(3) = 49 is a term because 49 = 7^2 is a semiprime with digits 4 and 9.
%p A368337 R:= 4,9:
%p A368337 for d from 2 to 6 do
%p A368337   for x from 0 to 2^d-1 do
%p A368337     L:= convert(2^d+x,base,2)[1..d];
%p A368337     y:= add((L[i]*5+4)*10^(i-1),i=1..d);
%p A368337     if numtheory:-bigomega(y)=2 then R:= R,y; fi
%p A368337 od od:
%p A368337 R;
%Y A368337 Intersection of A001358 and A284973.
%Y A368337 Cf. A020466.
%K A368337 nonn,base
%O A368337 1,1
%A A368337 _Zak Seidov_ and _Robert Israel_, Dec 21 2023