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A287932 a(n) = least k > n such that lpf(n) = lpf(k), where lpf = least prime factor (A020639). 2
4, 9, 6, 25, 8, 49, 10, 15, 12, 121, 14, 169, 16, 21, 18, 289, 20, 361, 22, 27, 24, 529, 26, 35, 28, 33, 30, 841, 32, 961, 34, 39, 36, 55, 38, 1369, 40, 45, 42, 1681, 44, 1849, 46, 51, 48, 2209, 50, 77, 52, 57, 54, 2809, 56, 65, 58, 63, 60, 3481, 62, 3721, 64 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

This sequence is a permutation of the composite numbers (A002808).

a(p) = p^2 for any prime p (see A001248).

a(2*k) = 2*k + 2 for any k > 1.

For any prime p and n >= 0, a^n(p)/p is the (n+1)-th p-rough number (where a^n denotes the n-th iterate of a).

See also A071830 for the largest prime factor equivalent.

LINKS

Rémy Sigrist, Table of n, a(n) for n = 2..10000

MATHEMATICA

lpf[n_] := FactorInteger[n][[1, 1]]; a[n_] := Block[{k, p = lpf[n]}, k=n+p; While[lpf[k] != p, k += p]; k]; Array[a, 61, 2] (* Giovanni Resta, Jun 04 2017 *)

PROG

(PARI) a(n) = my (l=factor(n)[1, 1]); forstep (v=n+l, oo, l, if (factor(v)[1, 1]==l, return (v)))

CROSSREFS

Cf. A002808, A020639, A071830.

A001248 is a subsequence.

Sequence in context: A061207 A140694 A152454 * A074767 A016097 A083717

Adjacent sequences:  A287929 A287930 A287931 * A287933 A287934 A287935

KEYWORD

nonn

AUTHOR

Rémy Sigrist, Jun 03 2017

STATUS

approved

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Last modified March 26 10:18 EDT 2019. Contains 321491 sequences. (Running on oeis4.)