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

1, 3, 6, 30, 35, 40, 45, 51, 57, 60, 66, 78, 87, 88, 93, 95, 102, 104, 105, 117, 121, 123, 136, 140, 143, 145, 156, 161, 174, 175, 185, 187, 203, 205, 215, 217, 219, 221, 232, 237, 245, 249, 258, 261, 267, 282, 285, 289, 291, 301, 303, 305, 321, 323, 325, 329

Offset: 1

Author

Michel Lagneau, Nov 24 2011

Keywords

• nonn

Comments

Omega=A001222: Number of prime divisors counted with multiplicity.

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.

Examples

			a(5) = 35 because Omega(35) = 2 and Omega(35 - 2) = Omega(33) = 2.
		

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001222.

Programs

Maple

with(numtheory):
isA200925 := proc(n)
        bigomega(n-bigomega(n)) = bigomega(n) ;
end proc:
for n from 1 to 400 do
        if isA200925(n) then printf("%d,",n) ; end if;
end do: # R. J. Mathar, Nov 28 2011

Mathematica

Select[Range[329], PrimeOmega[#] == PrimeOmega[# - PrimeOmega[#]] &] (* T. D. Noe, Nov 29 2011 *)