login
A125604
Minimum of the largest prime factors of a number and its two neighbors.
0
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
OFFSET
3,1
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
Sequence in context: A262941 A127656 A109709 * A216685 A331853 A187184
KEYWORD
nonn
AUTHOR
Carlos Alves, Nov 27 2006
STATUS
approved