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A078701 Least odd prime factor of n, or 1 if no such factor exists. 23
1, 1, 3, 1, 5, 3, 7, 1, 3, 5, 11, 3, 13, 7, 3, 1, 17, 3, 19, 5, 3, 11, 23, 3, 5, 13, 3, 7, 29, 3, 31, 1, 3, 17, 5, 3, 37, 19, 3, 5, 41, 3, 43, 11, 3, 23, 47, 3, 7, 5, 3, 13, 53, 3, 5, 7, 3, 29, 59, 3, 61, 31, 3, 1, 5, 3, 67, 17, 3, 5, 71, 3, 73, 37, 3, 19, 7, 3, 79, 5, 3, 41, 83, 3, 5, 43, 3, 11 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
LINKS
FORMULA
a(n) = A020639(A000265(n));
a(n) = A020639(n) iff n is odd; a(2^k) = 1.
MAPLE
A078701 := proc(n)
fs := numtheory[factorset](n) minus {2};
if fs = {} then
1;
else
min(op(fs)) ;
end if;
end proc: # R. J. Mathar, Feb 06 2019
MATHEMATICA
lof[n_]:=Module[{fac=Select[Transpose[FactorInteger[n]][[1]], OddQ]}, If[fac=={}, 1, Min[fac]]]; Array[lof, 90] (* Harvey P. Dale, Apr 14 2012 *)
a[n_] := FactorInteger[n/2^IntegerExponent[n, 2]][[1, 1]]; Array[a, 100] (* Amiram Eldar, Jul 04 2022 *)
PROG
(Haskell)
a078701 n = if null odds then 1 else head odds
where odds = tail $ a182469_row n
-- Reinhard Zumkeller, Oct 08 2012
(PARI) a(n) = my(v = select(x->((x%2)==1), factor(n)[, 1])); if (#v, vecmin(v), 1); \\ Michel Marcus, Oct 25 2017
(PARI) A078701(n)=iferr(factor(n)[2-bittest(n, 0), 1], E, 1) \\ M. F. Hasler, Nov 06 2017
(Python)
from sympy import factorint
def A078701(n): return min((p for p in factorint(n) if p > 2), default=1) # Chai Wah Wu, Feb 03 2022
CROSSREFS
Sequence in context: A308655 A356365 A217663 * A299766 A161398 A204455
KEYWORD
nonn
AUTHOR
Reinhard Zumkeller, Dec 18 2002
STATUS
approved

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Last modified April 21 14:30 EDT 2024. Contains 371874 sequences. (Running on oeis4.)