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A274364 Numbers n such that n and n+1 both have 28 divisors. 2

%I #8 Jan 10 2017 18:10:03

%S 156735,809919,1276479,1429568,1649727,1836351,2269376,2489535,

%T 2549312,2769471,2922560,3202496,3236031,4449087,4729023,4915647,

%U 5068736,5288895,5441984,5628608,5721920,6035391,6281792,6468416,6595263,6841664,7028288,7248447,7528383

%N Numbers n such that n and n+1 both have 28 divisors.

%H Charles R Greathouse IV, <a href="/A274364/b274364.txt">Table of n, a(n) for n = 1..10000</a>

%t SequencePosition[DivisorSigma[0,Range[7600000]],{28,28}][[All,1]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jan 10 2017 *)

%o (PARI) is(n)=numdiv(n)==28 && numdiv(n+1)==28

%o (PARI) list(lim)=my(v=List(),t,p6); forprime(p=2,sqrtnint(lim\6,6), p6=p^6; forprime(q=2,lim\p6\2, if(p==q,next); forprime(r=2,lim\p6\q, if(p==r || q==r, next); t=p6*q*r; if(numdiv(t+1)==28, listput(v,t)); if(numdiv(t-1)==28, listput(v,t-1))))); forprime(p=2,sqrtnint(lim\8,6), p6=p^6; forprime(q=2,sqrtnint(lim\p6,3), if(p==q, next); t=p6*q^3; if(numdiv(t+1)==28, listput(v,t));if(numdiv(t-1)==28, listput(v,t-1)))); Set(v)

%Y Intersection of A005237 and A137491.

%Y Cf. A000005, A274357.

%K nonn

%O 1,1

%A _Charles R Greathouse IV_, Jun 19 2016

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Last modified March 28 14:19 EDT 2024. Contains 371254 sequences. (Running on oeis4.)