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 A298268 a(1) = 1, and for any n > 1, if n is the k-th number with greatest prime factor p, then a(n) is the k-th number with least prime factor p. 2
 1, 2, 3, 4, 5, 9, 7, 6, 15, 25, 11, 21, 13, 49, 35, 8, 17, 27, 19, 55, 77, 121, 23, 33, 65, 169, 39, 91, 29, 85, 31, 10, 143, 289, 119, 45, 37, 361, 221, 95, 41, 133, 43, 187, 115, 529, 47, 51, 161, 125, 323, 247, 53, 57, 209, 203, 437, 841, 59, 145, 61, 961 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS This sequence is a permutation of the natural numbers, with inverse A298882. For any prime p and k > 0: - if s_p(k) is the k-th p-smooth number and r_p(k) is the k-th p-rough number, - then a(p * s_p(k)) = p * r_p(k), - for example: a(11 * A051038(k)) = 11 * A008364(k). LINKS Rémy Sigrist, Table of n, a(n) for n = 1..10000 Rémy Sigrist, Colored logarithmic scatterplot of the first 100000 terms (with prime and semiprime values highlighted) Rémy Sigrist, Colored logarithmic scatterplot of the first 100000 terms (where the color is function of A006530(n)) Rémy Sigrist, Colored logarithmic scatterplot of the first 100000 terms (where the color is function of A176506(k) when a(n) is the k-th semiprime) Rémy Sigrist, PARI program for A298268 FORMULA a(1) = 1. a(A125624(n, k)) = A083140(n, k) for any n > 0 and k > 0. a(n) = A083140(A061395(n), A078899(n)) for any n > 1. Empirically: - a(n) = n iff n belongs to A046022, - a(2^k) = 2 * k for any k > 0, - a(2 * p) = p^2 for any prime p, - a(3 * p) = p * A151800(p) for any odd prime p. EXAMPLE The first terms, alongside A006530(n), are:   n     a(n)   gpf(n)   --    ----   ------    1      1      1    2      2      2    3      3      3    4      4      2    5      5      5    6      9      3    7      7      7    8      6      2    9     15      3   10     25      5   11     11     11   12     21      3   13     13     13   14     49      7   15     35      5   16      8      2   17     17     17   18     27      3   19     19     19   20     55      5 PROG (PARI) See Links section. CROSSREFS Cf. A006530, A008364, A046022, A051038, A061395, A078899, A083140, A125624, A151800, A176506, A298268, A298882 (inverse). Sequence in context: A262692 A245821 A075161 * A329044 A222251 A241916 Adjacent sequences:  A298265 A298266 A298267 * A298269 A298270 A298271 KEYWORD nonn AUTHOR Rémy Sigrist, Jan 27 2018 STATUS approved

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Last modified August 8 02:22 EDT 2020. Contains 336290 sequences. (Running on oeis4.)