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A128428 Number of distinct prime factors of n^2+1. 8
1, 1, 2, 1, 2, 1, 2, 2, 2, 1, 2, 2, 3, 1, 2, 1, 3, 2, 2, 1, 3, 2, 3, 1, 2, 1, 3, 2, 2, 2, 3, 2, 3, 2, 2, 1, 3, 2, 2, 1, 2, 2, 3, 2, 2, 2, 4, 2, 2, 2, 2, 2, 3, 1, 3, 1, 3, 2, 2, 2, 2, 2, 3, 2, 2, 1, 3, 2, 2, 2, 2, 3, 4, 1, 3, 2, 3, 2, 2, 2, 3, 2, 4, 1, 2, 2, 3, 2, 3, 1, 3, 2, 3, 1, 2, 2, 3, 3, 3, 2, 2, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

a(n) is also the number of distinct prime factors, up to multiplication by units, of n + i in the ring of Gaussian integers. - Jason Kimberley, Dec 17 2011

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

FORMULA

a(n) = A001221(n^2+1).

EXAMPLE

a(3) = 2 because 3^2+1 = 2*5.

MAPLE

a:= n-> nops(select(isprime, numtheory[divisors](n^2+1))):

seq(a(n), n=1..100);  # Alois P. Heinz, Dec 06 2020

MATHEMATICA

a[n_]:=Length[FactorInteger[n^2 + 1]]

PROG

(PARI) a(n)=omega(n^2+1) \\ Charles R Greathouse IV, Jul 31 2011

CROSSREFS

Cf. A193330 (counted with multiplicity).

Sequence in context: A329320 A316112 A317994 * A056171 A333749 A238949

Adjacent sequences:  A128425 A128426 A128427 * A128429 A128430 A128431

KEYWORD

nonn

AUTHOR

Kent Horvath (kenthorvath(AT)gmail.com), May 10 2007

STATUS

approved

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Last modified June 16 04:56 EDT 2021. Contains 345056 sequences. (Running on oeis4.)