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 A216784 a(n) is the number of prime divisors of n^2 + 1 of the form a^2 + 1. 1
 1, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 3, 1, 1, 1, 2, 1, 1, 1, 2, 1, 2, 1, 1, 1, 2, 1, 1, 1, 2, 1, 2, 0, 1, 1, 2, 2, 1, 1, 1, 1, 3, 0, 1, 0, 3, 1, 1, 0, 1, 1, 2, 1, 2, 1, 2, 1, 1, 0, 1, 1, 2, 1, 1, 1, 2, 2, 1, 0, 1, 2, 2, 1, 1, 0, 2, 1, 1, 1, 2, 1, 2, 1, 1, 0, 2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS a(m) = 0 for m = A217276(n). LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE a(13) = 3 because 13^2+1 = 170 = 2*5*17 with 3 divisors of the form a^2+1 such that 2 = 1^2+1, 5=2^2+1 and 17 = 4^2+1. a(34) = 0 because 34^2+1 = 1157 = 13*89 and the prime divisors 13, 89 are not of the form a^2+1. MAPLE with(numtheory):for n from 1 to 100 do:x:=n^2+1:y:=factorset(x):n1:=nops(y):i:=0:for k from 1 to n1 do:z:=sqrt(y[k]-1):if z=floor(z) then i:=i+1:else fi:od: printf(`%d, `, i):od: MATHEMATICA a[n_] := Length @ Select[FactorInteger[n^2 + 1][[;; , 1]], IntegerQ @ Sqrt[# - 1] &]; Array[a, 100] (* Amiram Eldar, Sep 11 2019 *) CROSSREFS Cf. A217276, A181436, A180252. Sequence in context: A331591 A003649 A287170 * A256067 A256554 A321649 Adjacent sequences:  A216781 A216782 A216783 * A216785 A216786 A216787 KEYWORD nonn AUTHOR Michel Lagneau, Oct 15 2012 STATUS approved

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Last modified June 21 17:44 EDT 2021. Contains 345365 sequences. (Running on oeis4.)