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A085986
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Squares of the squarefree semiprimes (p^2*q^2).
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43
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36, 100, 196, 225, 441, 484, 676, 1089, 1156, 1225, 1444, 1521, 2116, 2601, 3025, 3249, 3364, 3844, 4225, 4761, 5476, 5929, 6724, 7225, 7396, 7569, 8281, 8649, 8836, 9025, 11236, 12321, 13225, 13924, 14161, 14884, 15129, 16641, 17689, 17956, 19881
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refs;
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history;
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internal format)
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OFFSET
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1,1
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COMMENTS
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This sequence is a member of a family of sequences directly related to A025487. First terms and known sequences are listed below: 1, A000007; 2, A000040; 4, A001248; 6, A006881; 8, A030078; 12, A054753; 16, A030514; 24, A065036; 30, A007304; 32, A050997; 36, this sequence; 48, ?; 60, ?; 64, ?; ....
a(4)-a(3)=29 and a(3)+a(4)=421 are both prime. There are no other cases where the sum and difference of two members of this sequence are both prime. - Robert Israel and J. M. Bergot, Oct 25 2019
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LINKS
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FORMULA
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Sum_{n>=1} 1/a(n) = (P(2)^2 - P(4))/2 = (A085548^2 - A085964)/2 = 0.063767..., where P is the prime zeta function. - Amiram Eldar, Jul 06 2020
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EXAMPLE
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A006881 begins 6 10 14 15 ... so this sequence begins 36 100 196 225 ...
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MATHEMATICA
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Select[Range[200], PrimeOmega[#]==2&&SquareFreeQ[#]&]^2 (* Harvey P. Dale, Mar 07 2013 *)
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PROG
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(PARI) list(lim)=my(v=List(), x=sqrtint(lim\=1), t); forprime(p=2, x\2, t=p; forprime(q=2, min(x\t, p-1), listput(v, (t*q)^2))); Set(v) \\ Charles R Greathouse IV, Sep 22 2015
(Magma) [k^2:k in [1..150]| IsSquarefree(k) and #PrimeDivisors(k) eq 2]; // Marius A. Burtea, Oct 24 2019
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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