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A196941 a(n) is the minimum prime (or 1) needed to write integer n into the form n = a + b such that all prime factors of a and b are smaller or equal to a(n). 2
1, 2, 2, 2, 2, 3, 2, 2, 2, 3, 2, 3, 3, 3, 2, 2, 2, 3, 2, 3, 3, 5, 2, 3, 3, 3, 3, 3, 3, 3, 2, 2, 2, 3, 2, 3, 3, 3, 2, 3, 3, 3, 3, 3, 5, 5, 2, 3, 3, 3, 3, 5, 3, 3, 3, 3, 3, 3, 3, 5, 3, 3, 2, 2, 2, 3, 2, 5, 3, 7, 2, 3, 3, 3, 3, 5, 3, 5, 2, 3, 3, 3, 3, 3, 3, 3, 3 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,2

COMMENTS

Any integer n that is greater than 1 can be written into the sum of two other positive integers, such that n = a + b.  There are IntegerPart[n / 2] ways to do this assuming a <= b.  For each of the ways, we can have a set of prime factor of a and b, defined as sa = FactorSet[a] and sb = FactorSet[b], quoting the function in the Mathematica program.  Then we can define a union set s=Union[sa, sb], which is a list of prime factors that can factor either a or b.  In this way we obtain IntegerPart[n / 2] of possible set s.  Define p_i is the largest prime number in each of set s_i, i = 1,2...IntegerPart[n / 2], a(n) = the smallest s_i.

Though 2 =  1 + 1 and 1 is not a prime number, a(2) can still be defined as 1.

The Mathematica program generates up to term 88.

The first occurence of a(n)=k forms sequence A000229. - Lei Zhou, Feb 06 2014

LINKS

T. D. Noe, Table of n, a(n) for n = 2..10000

EXAMPLE

n = 3, 3 = 1 + 2, the largest prime factor of 1 and 2 is 2, so a[3] = 2;

n = 4, 4 = 2 + 2, the largest prime factor of 2 and 2 is 2, so a[4] = 2;

[in 4 = 1 + 3, the largest prime factor of 1 and 3 is 3, which is larger than a[4] = 2]

...

n = 23, 23 = 3 + 20 = 3 + 2^2 * 5, the largest prime factor of 3 and 20 is 5, so a[23] = 5;

MATHEMATICA

FactorSet[seed_] := Module[{fset2, a, l, i}, a = FactorInteger[seed]; l = Length[a]; fset2 = {}; Do[fset2 = Union[fset2, {a[[i]][[1]]}], {i, 1, l}]; fset2]; Table[min = n; Do[r = n - k; s = Union[FactorSet[k], FactorSet[r]]; If[a = s[[Length[s]]]; a < min, min = a], {k, 1, IntegerPart[n/2]}]; min, {n, 2, 88}]

LPF[n_] := FactorInteger[n][[-1, 1]]; Table[Min[Table[Max[{LPF[i], LPF[n-i]}], {i, Floor[n/2]}]], {n, 2, 100}] (* T. D. Noe, Oct 07 2011 *)

CROSSREFS

Cf. A173786 (n for which a(n)=2), A196526, A000229.

Sequence in context: A294903 A087133 A324292 * A062843 A136164 A182860

Adjacent sequences:  A196938 A196939 A196940 * A196942 A196943 A196944

KEYWORD

nonn,easy

AUTHOR

Lei Zhou, Oct 07 2011

STATUS

approved

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Last modified September 20 22:20 EDT 2019. Contains 327252 sequences. (Running on oeis4.)