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A179690 Numbers of the form p^2*q^2*r*s where p, q, r, and s are distinct primes. 7


%S 1260,1980,2100,2340,2772,2940,3060,3150,3276,3300,3420,3900,4140,

%T 4284,4410,4788,4950,5100,5148,5220,5580,5700,5796,5850,6468,6660,

%U 6732,6900,7260,7308,7350,7380,7524,7644,7650,7700,7740,7812,7956,8460,8550,8700

%N Numbers of the form p^2*q^2*r*s where p, q, r, and s are distinct primes.

%H T. D. Noe, <a href="/A179690/b179690.txt">Table of n, a(n) for n = 1..1000</a>

%H Will Nicholes, <a href="http://willnicholes.com/math/primesiglist.htm">Prime Signatures</a>

%H <a href="/index/Pri#prime_signature">Index to sequences related to prime signature</a>

%p op(select(n->nops(factorset(n))=4 and sort([seq(op(2,a),a=ifactors(n)[2])])=[1,1,2,2],[$1..8700])); # _Paolo P. Lava_, Jul 18 2019

%t f[n_]:=Sort[Last/@FactorInteger[n]]=={1,1,2,2}; Select[Range[10000], f]

%o (PARI) list(lim)=my(v=List(),t1,t2,t3); forprime(p=2,sqrtint(lim\60), t1=p^2; forprime(q=2,sqrtint(lim\(6*t1)), if(q==p, next); t2=q^2*t1; forprime(r=2,lim\(2*t2), if(r==p || r==q, next); t3=r*t2; forprime(s=2,lim\t3, if(s==p || s==q || s==r, next); listput(v, t3*s))))); Set(v) \\ _Charles R Greathouse IV_, Aug 25 2016

%Y Part of the list A178739 .. A179696 and A030514 .. A030629, A189975 .. A189990 etc., cf. A101296. - _M. F. Hasler_, Jul 17 2019

%K nonn

%O 1,1

%A _Vladimir Joseph Stephan Orlovsky_, Jul 24 2010

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Last modified October 17 08:36 EDT 2019. Contains 328107 sequences. (Running on oeis4.)