cp's OEIS Frontend

A200925 Numbers k such that Omega(k) = Omega(k - Omega(k)), where Omega = A001222.

%I A200925 #22 Jul 14 2025 13:07:40
%S A200925 1,3,6,30,35,40,45,51,57,60,66,78,87,88,93,95,102,104,105,117,121,123,
%T A200925 136,140,143,145,156,161,174,175,185,187,203,205,215,217,219,221,232,
%U A200925 237,245,249,258,261,267,282,285,289,291,301,303,305,321,323,325,329
%N A200925 Numbers k such that Omega(k) = Omega(k - Omega(k)), where Omega = A001222.
%C A200925 Omega=A001222: Number of prime divisors counted with multiplicity.
%C A200925 A198327 is a subsequence because, if k and k-2 are semiprimes, Omega(k) = 2, and k - 2 is semiprime, therefore Omega(k-2) = 2.
%H A200925 Vincenzo Librandi, <a href="/A200925/b200925.txt">Table of n, a(n) for n = 1..10000</a>
%e A200925 a(5) = 35 because Omega(35) = 2 and Omega(35 - 2) = Omega(33) = 2.
%p A200925 with(numtheory):
%p A200925 isA200925 := proc(n)
%p A200925         bigomega(n-bigomega(n)) = bigomega(n) ;
%p A200925 end proc:
%p A200925 for n from 1 to 400 do
%p A200925         if isA200925(n) then printf("%d,",n) ; end if;
%p A200925 end do: # _R. J. Mathar_, Nov 28 2011
%t A200925 Select[Range[329], PrimeOmega[#] == PrimeOmega[# - PrimeOmega[#]] &] (* _T. D. Noe_, Nov 29 2011 *)
%Y A200925 Cf. A001222.
%K A200925 nonn
%O A200925 1,2
%A A200925 _Michel Lagneau_, Nov 24 2011