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A143868 a(n)= Number of distinct prime divisors (taken together) of numbers of the form 2x^2-1 for x<=10^n. 4
8, 84, 815, 7922, 77250, 759077, 7492588, 74198995, 736401956, 7319543972, 72834161468, 725344237597 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..12.

Bernhard Helmes, Prime sieving on the polynomial f(n)=2n^2-1.

MATHEMATICA

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*)

CROSSREFS

Cf. A143835.

Sequence in context: A039939 A199907 A264710 * A130591 A048665 A005797

Adjacent sequences:  A143865 A143866 A143867 * A143869 A143870 A143871

KEYWORD

nonn

AUTHOR

Artur Jasinski, Sep 04 2008

EXTENSIONS

Fixed broken link and corrected terms to agree with website - Ray Chandler, Jun 30 2015

STATUS

approved

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Last modified July 9 22:46 EDT 2020. Contains 335570 sequences. (Running on oeis4.)