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A256970 Smallest prime divisor of 4*n^2+1. 5
5, 17, 37, 5, 101, 5, 197, 257, 5, 401, 5, 577, 677, 5, 17, 5, 13, 1297, 5, 1601, 5, 13, 29, 5, 41, 5, 2917, 3137, 5, 13, 5, 17, 4357, 5, 13, 5, 5477, 53, 5, 37, 5, 7057, 13, 5, 8101, 5, 8837, 13, 5, 73, 5, 29, 17, 5, 12101, 5, 41, 13457, 5 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
a(n) = A020639(A053755(n)).
If the map "x -> smallest odd prime divisor of n^2+1" is iterated, does it always terminate in the 2-cycle (5 <-> 13)? - Zoran Sunic, Oct 25 2017
REFERENCES
Richard Friedberg, An Adventurer's Guide to Number Theory, McGraw-Hill, NY, 1968.
Popular Computing (Calabasas, CA), Friedberg's Sequence, Vol. 5 (No. 46, Jan 1977), page PC46-2.
LINKS
MATHEMATICA
Table[FactorInteger[4*n^2+1][[1, 1]], {n, 59}] (* Ivan N. Ianakiev, Apr 20 2015 *)
PROG
(Haskell)
a256970 = a020639 . a053755 -- Reinhard Zumkeller, Apr 20 2015
(PARI) a(n) = factor(4*n^2+1)[1, 1]; \\ Michel Marcus, Apr 20 2015
CROSSREFS
A bisection of A125256.
Sequence in context: A083048 A123669 A121325 * A145689 A321658 A080167
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Apr 19 2015
STATUS
approved

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