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A117365
a(n) = largest prime less than the smallest prime dividing n (or 1 if there is no such prime).
5
1, 1, 2, 1, 3, 1, 5, 1, 2, 1, 7, 1, 11, 1, 2, 1, 13, 1, 17, 1, 2, 1, 19, 1, 3, 1, 2, 1, 23, 1, 29, 1, 2, 1, 3, 1, 31, 1, 2, 1, 37, 1, 41, 1, 2, 1, 43, 1, 5, 1, 2, 1, 47, 1, 3, 1, 2, 1, 53, 1, 59, 1, 2, 1, 3, 1, 61, 1, 2, 1, 67, 1, 71, 1, 2, 1, 5, 1, 73, 1, 2, 1, 79, 1, 3, 1, 2, 1, 83, 1, 5, 1, 2, 1, 3, 1
OFFSET
1,3
COMMENTS
a(n) = 1 if and only if n is even or if n = 1.
LINKS
EXAMPLE
5 is the smallest prime dividing 35. So a(35) is the largest prime < 5, which is 3.
MATHEMATICA
a[n_] := If[EvenQ[n], 1, Prime[PrimePi[FactorInteger[n][[1]][[1]]] - 1]]; Table[a[n], {n, 2, 80}] (* Stefan Steinerberger, Mar 14 2006 *)
Table[NextPrime[FactorInteger[n][[1, 1]], -1] /. -2 -> 1, {n, 96}] (* Michael De Vlieger, Sep 16 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Leroy Quet, Mar 10 2006
EXTENSIONS
More terms from Stefan Steinerberger, Mar 14 2006
More terms from Franklin T. Adams-Watters, Aug 29 2006
STATUS
approved