The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A085986 Squares of the squarefree semiprimes (p^2*q^2). 35
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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, ?; .... Subsequence of A077448. The numbers in A077448 but not in here are 1, the squares of A046386, the squares of A067885, etc. - R. J. Mathar, Sep 12 2008 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 LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 FORMULA a(n) = A006881(n)^2. 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 EXAMPLE A006881 begins 6 10 14 15 ... so this sequence begins 36 100 196 225 ... MAPLE with(numtheory): P:=proc(n) if nops(factorset(n))=2 and bigomega(n)=2 then n^2; fi; end: seq(P(i), i=1..141); # Paolo P. Lava, Jul 18 2019 MATHEMATICA f[n_]:=Sort[Last/@FactorInteger[n]]=={2, 2}; Select[Range, f] (* Vladimir Joseph Stephan Orlovsky, Aug 14 2009 *) Select[Range, PrimeOmega[#]==2&&SquareFreeQ[#]&]^2 (* Harvey P. Dale, Mar 07 2013 *) PROG (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 (PARI) is(n)=factor(n)[, 2]==[2, 2]~ \\ Charles R Greathouse IV, Oct 19 2015 (MAGMA) [k^2:k in [1..150]| IsSquarefree(k) and #PrimeDivisors(k) eq 2]; // Marius A. Burtea, Oct 24 2019 CROSSREFS Subsequence of A036785 and of A077448. Subsequence of A062503. Cf. A025487. Cf. A085548, A085964. Sequence in context: A030627 A177492 A077448 * A027603 A250813 A268770 Adjacent sequences:  A085983 A085984 A085985 * A085987 A085988 A085989 KEYWORD easy,nonn AUTHOR Alford Arnold, Jul 06 2003 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified October 24 02:30 EDT 2020. Contains 337975 sequences. (Running on oeis4.)