A125604 - OEIS

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A125604

Minimum of the largest prime factors of a number and its two neighbors.

2, 2, 2, 3, 2, 2, 2, 3, 3, 3, 3, 5, 2, 2, 2, 3, 3, 5, 5, 7, 3, 3, 3, 3, 3, 3, 5, 5, 2, 2, 2, 7, 3, 3, 3, 13, 5, 5, 5, 7, 7, 5, 5, 5, 3, 3, 3, 5, 5, 13, 3, 3, 3, 7, 7, 19, 5, 5, 5, 7, 2, 2, 2, 11, 11, 17, 7, 7, 3, 3, 3, 5, 5, 5, 11, 11, 5, 3, 3, 3, 7, 7, 7, 17, 11, 11, 5, 5, 5, 13, 23, 19, 3, 3, 3, 7, 5, 5

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OFFSET

3,1

LINKS

Table of n, a(n) for n=3..100.

FORMULA

a(n) = min{lpf(n-1),lpf(n),lpf(n+1)}, where lpf is the largest prime factor: lpf(k) = A006530(k).

EXAMPLE

a(93) = min{lpf(92),lpf(93),lpf(94)} = min{23,31,47} = 23.

MATHEMATICA

LPF = FactorInteger[ # ][[ -1, 1]] &; Map[Min[{LPF[ # - 1], LPF[ # ], LPF[ # + 1]}] &, Range[3, 200]]

Min/@Partition[Table[FactorInteger[n][[-1, 1]], {n, 110}], 3, 1] (* Harvey P. Dale, May 25 2015 *)

PROG

(PARI) a(n) = my(lpf(k)=vecmax(factor(k)[, 1])); vecmin([lpf(n-1), lpf(n), lpf(n+1)]); \\ Ruud H.G. van Tol, Aug 15 2024

CROSSREFS

Cf. A006530, A093074.

Sequence in context: A262941 A127656 A109709 * A216685 A331853 A187184

Adjacent sequences: A125601 A125602 A125603 * A125605 A125606 A125607

KEYWORD

nonn

AUTHOR

Carlos Alves, Nov 27 2006

STATUS

approved