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 A064911 #67 Jul 22 2025 20:57:55
%S A064911 0,0,0,1,0,1,0,0,1,1,0,0,0,1,1,0,0,0,0,0,1,1,0,0,1,1,0,0,0,0,0,0,1,1,
%T A064911 1,0,0,1,1,0,0,0,0,0,0,1,0,0,1,0,1,0,0,0,1,0,1,1,0,0,0,1,0,0,1,0,0,0,
%U A064911 1,0,0,0,0,1,0,0,1,0,0,0,0,1,0,0,1,1,1,0,0,0,1,0,1,1,1,0,0,0,0,0,0,0,0,0,0
%N A064911 If n is semiprime (or 2-almost prime) then 1 else 0.
%H A064911 Antti Karttunen, <a href="/A064911/b064911.txt">Table of n, a(n) for n = 1..65545</a> (first 10000 terms from Reinhard Zumkeller)
%H A064911 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/Semiprime.html">Semiprime</a>
%H A064911 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PrimeZetaFunction.html">Prime zeta function primezeta(s)</a>.
%H A064911 <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>
%H A064911 <a href="/index/Eu#epf">Index entries for sequences computed from exponents in factorization of n</a>
%F A064911 a(n) = 1 iff n is in A001358 (semiprimes), a(n) = 0 iff n is in A100959 (non-semiprimes). - _Reinhard Zumkeller_, Nov 24 2004
%F A064911 Dirichlet g.f.: (primezeta(2s) + primezeta(s)^2)/2. - _Franklin T. Adams-Watters_, Jun 09 2006
%F A064911 a(n) = A057427(A174956(n)); a(n)*A072000(n) = A174956(n). - _Reinhard Zumkeller_, Apr 03 2010
%F A064911 a(n) = A010051(A032742(n)) (i.e., largest proper divisor is prime). - _Reinhard Zumkeller_, Mar 13 2011
%F A064911 From _Antti Karttunen_, Apr 24 2018 & Apr 22 2022: (Start)
%F A064911 a(n) = A280710(n) + A302048(n) = A101040(n) - A010051(n).
%F A064911 a(n) = A353478(n) + A353480(n) = A353477(n) + A353478(n) + A353479(n).
%F A064911 a(n) = A353475(n) + A353476(n).
%F A064911 (End)
%F A064911 a(n) = [Omega(n) = 2], where [ ] is the Iverson bracket. - _Wesley Ivan Hurt_, Jul 22 2025
%p A064911 with(numtheory):
%p A064911 a:= n-> `if`(bigomega(n)=2, 1, 0):
%p A064911 seq(a(n), n=1..120); # _Alois P. Heinz_, Mar 16 2011
%t A064911 Table[If[PrimeOmega[n] == 2, 1, 0], {n, 105}] (* _Jayanta Basu_, May 25 2013 *)
%o A064911 (Haskell) a064911 = a010051 . a032742 -- _Reinhard Zumkeller_, Mar 13 2011
%o A064911 (PARI) a(n)=bigomega(n)==2 \\ _Charles R Greathouse IV_, Mar 13 2011
%Y A064911 Cf. A010051, A064899-A064910, A053409, A046413, A101040, A105700, A280710, A302047, A302048, A302049, A353475, A353476, A353477, A353478, A353479, A353480, A353481.
%K A064911 nonn
%O A064911 1,1
%A A064911 _Patrick De Geest_, Oct 13 2001
%E A064911 Edited by _M. F. Hasler_, Oct 18 2017