A217048 - OEIS

%I #16 Sep 08 2022 08:46:04

%S 6,9,33,38,39,69,86,93,303,309,339,386,393,398,633,669,689,698,699,

%T 803,838,866,869,886,889,893,898,899,933,939,989,993,998,3039,3063,

%U 3086,3093,3098,3099,3309,3338,3369,3383,3386,3398,3603,3639,3669,3683,3693

%N Semiprimes using only the curved digits 0, 3, 6, 8 and 9.

%C This is to A079652 as semiprimes A001358 are to primes A000040.

%H Robert Israel, <a href="/A217048/b217048.txt">Table of n, a(n) for n = 1..10000</a>

%F A001358 INTERSECTION A072960.

%e a(41) = 3338 = 2 * 1669, the 938th semiprime.

%p R:= [0,3,6,8,9]:

%p Res:= NULL: count:= 0:

%p for m from 1 while count < 100 do

%p   L:= convert(m,base,5);

%p   n:= add(R[L[i]+1]*10^(i-1),i=1..nops(L));

%p   if numtheory:-bigomega(n)=2 then Res:= Res, n; count:= count+1 fi

%p od:

%p Res; # _Robert Israel_, Feb 16 2020

%o (Magma) IsSemiprime:=func<n | &+[d[2]: d in Factorization(n)] eq 2>; [n: n in [2..3700] | IsSemiprime(n) and Intseq(n) subset [0,3,6,8,9]]; // _Bruno Berselli_, Sep 25 2012

%Y Cf. A001358, A072960.

%K nonn,base,easy

%O 1,1

%A _Jonathan Vos Post_, Sep 25 2012