This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A210615 #26 May 15 2025 22:11:28
%S A210615 0,0,0,4,0,6,0,4,9,10,0,4,0,14,15,4,0,6,0,4,21,22,0,4,25,26,9,4,0,6,0,
%T A210615 4,33,34,35,4,0,38,39,4,0,6,0,4,9,46,0,4,49,10,51,4,0,6,55,4,57,58,0,
%U A210615 4,0,62,9,4,65,6,0,4,69,10,0,4,0,74,15,4,77,6
%N A210615 Least semiprime dividing n, or 0 if no semiprime divides n.
%C A210615 Roughly analogous to Least Prime Factor A020639 but with semiprimes rather than primes.
%H A210615 T. D. Noe, <a href="/A210615/b210615.txt">Table of n, a(n) for n = 1..1000</a>
%F A210615 a(n) = min {k such that k|n and k in A001358} else 0 if there exists no such k.
%F A210615 a(p) = 0 iff p in A008578. - _Alois P. Heinz_, Mar 28 2012
%e A210615 a(24) = 4 because 24 is divisible by the semiprimes {4,6} of which 4 is the smallest.
%t A210615 Table[If[PrimeQ[n] || n < 2, 0, f = FactorInteger[n]; If[f[[1,2]] > 1, f[[1,1]]^2, f[[1,1]]*f[[2,1]]]], {n, 100}] (* _T. D. Noe_, Mar 24 2012 *)
%t A210615 Flatten[Table[Select[Divisors[n],PrimeOmega[#]==2&,1],{n,80}]/.{}->{0}] (* _Harvey P. Dale_, Dec 07 2012 *)
%Y A210615 Cf. A001358, A008578, A020639, A088739 (this sequence without the zeros).
%K A210615 nonn,easy
%O A210615 1,4
%A A210615 _Jonathan Vos Post_, Mar 23 2012