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 A101637 #23 Feb 16 2025 08:32:55
%S A101637 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,
%T A101637 0,1,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,1,0,0,0,1,0,0,0,0,0,0,0,0,
%U A101637 0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,1,0,0,0,1,0,1,0,0,0,0,0,0,0,0,0,1
%N A101637 a(n) = 1 if n is a 4-almost prime, that is a product of exactly four (not necessarily distinct) primes, 0 otherwise.
%C A101637 Characteristic function of A014613.
%C A101637 See A101638 for the inverse Moebius transform of this sequence.
%H A101637 Antti Karttunen, <a href="/A101637/b101637.txt">Table of n, a(n) for n = 1..65537</a>
%H A101637 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Almostprime.html">Almost Prime.</a>
%H A101637 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>
%H A101637 <a href="/index/Eu#epf">Index entries for sequences computed from exponents in factorization of n</a>
%e A101637 a(100) = 1 because 100 = 2 * 2 * 5 * 5 is the product of exactly 4 primes and thus is a 4-almost prime.
%t A101637 Table[If[PrimeOmega[n]==4,1,0],{n,100}] (* _Harvey P. Dale_, Sep 13 2024 *)
%o A101637 (PARI) a(n)=bigomega(n)==4 \\ _Charles R Greathouse IV_, Jan 31 2017
%Y A101637 Cf. A101638, A014613, A000040, A001358, A014612, A014614.
%K A101637 easy,nonn
%O A101637 1,1
%A A101637 _Jonathan Vos Post_, Dec 10 2004
%E A101637 Name edited by _Antti Karttunen_, Oct 08 2017