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A078701
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Least odd prime factor of n, or 1 if no such factor exists.
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24
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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
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OFFSET
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1,3
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LINKS
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FORMULA
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a(n) = A020639(n) iff n is odd; a(2^k) = 1.
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MAPLE
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fs := numtheory[factorset](n) minus {2};
if fs = {} then
1;
else
min(op(fs)) ;
end if;
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MATHEMATICA
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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 *)
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PROG
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(Haskell)
a078701 n = if null odds then 1 else head odds
where odds = tail $ a182469_row n
(PARI) a(n) = my(v = select(x->((x%2)==1), factor(n)[, 1])); if (#v, vecmin(v), 1); \\ Michel Marcus, Oct 25 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
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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