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A075730 Squares of odd semiprimes A046315, odd numbers divisible by exactly 2 primes (counted with multiplicity). 1
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified May 8 06:39 EDT 2021. Contains 343653 sequences. (Running on oeis4.)