A075730 Squares of odd semiprimes A046315, odd numbers divisible by exactly 2 primes (counted with multiplicity).

81, 225, 441, 625, 1089, 1225, 1521, 2401, 2601, 3025, 3249, 4225, 4761, 5929, 7225, 7569, 8281, 8649, 9025, 12321, 13225, 14161, 14641, 15129, 16641, 17689, 19881, 20449, 21025, 24025, 25281, 25921, 28561, 31329, 33489, 34225, 34969, 40401

Offset

1,1

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Formula

Sum_{n>=1} 1/a(n) = P(2)^2/2 + P(4)/2 - P(2)/4 = 0.02769857933..., where P is the prime zeta function. - Amiram Eldar, Mar 22 2021

Example

9 is odd and divisible by 3 (twice) and 9*9=81.
15 is odd and divisible by 3 and 5 and 15*15=225.

Maple

readlib(issqr): ts_kv_sp_lih := proc(n); if (numtheory[bigomega](n)=4 and type(n, odd)='true' and issqr(n)='true') then RETURN(n); fi; end: seq(ts_kv_sp_lih(i), i=1..100000);

Mathematica

Select[Range[1, 201, 2], PrimeOmega[#] == 2 &]^2 (* Amiram Eldar, Mar 22 2021 *)

Crossrefs

Equals A046315(n)^2.

Sequence in context: A236721 A255110 A062532 * A184068 A211690 A205055

Adjacent sequences: A075727 A075728 A075729 * A075731 A075732 A075733

Keyword

easy,nonn

Author

Jani Melik, Oct 07 2002

Extensions

Checked by Zak Seidov, Mar 08 2006

Status

approved