A178065 Number of 1's in binary representation of n-th semiprime.

1, 2, 2, 2, 3, 4, 3, 3, 3, 3, 2, 2, 3, 3, 4, 4, 3, 4, 5, 4, 4, 5, 2, 3, 3, 4, 3, 4, 4, 5, 5, 5, 5, 6, 4, 6, 5, 5, 6, 5, 5, 6, 2, 3, 3, 4, 4, 5, 3, 3, 5, 5, 6, 3, 4, 4, 4, 4, 6, 5, 6, 3, 4, 4, 5, 5, 5, 4, 5, 5, 6, 5, 5, 6, 6, 4, 6, 6, 7, 6, 7, 7, 3, 3, 3, 4, 3, 4, 6, 3, 4, 5, 4, 5, 5, 5, 6, 4, 5, 5, 7, 3, 4, 4, 5

Offset

1,2

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

a(1)=1 because the first semiprime is 4 = 100_2.
a(2)=2 because the second semiprime is 6 = 110_2.

Maple

N:= 10^3: # for semiprimes <= N
P:= select(isprime, [2, seq(i, i=3..N/2, 2)]):
SP:= sort(select(`<=`, [seq(seq(P[i]*P[j], j=1..i), i=1..nops(P))], N)):
map(t -> convert(convert(t, base, 2), `+`), SP); # Robert Israel, Feb 02 2025

Mathematica

s={}; Do[If[PrimeOmega[n]==2, AppendTo[s, Total[IntegerDigits[n, 2]]]], {n, 400}]; s (* James C. McMahon, Jan 02 2025 *)

Prog

(PARI) lista(nn) = {for (n=2, nn, if (bigomega(n)==2, print1(norml2(binary(n)), ", ")); ); } \\ Michel Marcus, Jun 05 2013

Crossrefs

Cf. A001358, A014499, A102029.

Sequence in context: A281945 A163107 A275840 * A366636 A366655 A057748

Adjacent sequences: A178062 A178063 A178064 * A178066 A178067 A178068

Keyword

nonn,base,easy

Author

Juri-Stepan Gerasimov, May 18 2010

Extensions

a(56) corrected by R. J. Mathar, May 23 2010

Status

approved