A143868 - OEIS

A143868

a(n)= Number of distinct prime divisors (taken together) of numbers of the form 2x^2-1 for x<=10^n.

%I #7 Jun 30 2015 13:21:30

%S 8,84,815,7922,77250,759077,7492588,74198995,736401956,7319543972,

%T 72834161468,725344237597

%N a(n)= Number of distinct prime divisors (taken together) of numbers of the form 2x^2-1 for x<=10^n.

%H Bernhard Helmes, <a href="http://www.devalco.de/quadr_Sieb_2x%5E2-1.php#4a">Prime sieving on the polynomial f(n)=2n^2-1</a>.

%t l = 0; p = 2; c = {}; a = {}; Do[k = p x^2 - 1; b = Divisors[k]; Do[If[PrimeQ[b[[n]]], AppendTo[a, b[[n]]]], {n, 1, Length[b]}]; If[N[Log[x]/Log[10]] == Round[N[Log[x]/Log[10]]], a = Union[a]; l = Length[a]; Print[l]; AppendTo[c, l]], {x, 1, 10000}]; c (*Artur Jasinski*)

%Y Cf. A143835.

%K nonn

%O 1,1

%A _Artur Jasinski_, Sep 04 2008

%E Fixed broken link and corrected terms to agree with website - _Ray Chandler_, Jun 30 2015