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 A330432 Number of permutations sigma of [n] such that k * sigma(k) >= n for 1 <= k <= n. 2
 1, 1, 1, 1, 2, 4, 18, 54, 384, 1920, 15000, 108000, 1270080, 8890560, 126443520, 1440270720, 18811699200, 232243200000, 4303470124800, 56336336179200, 1165698293760000, 17784994106880000, 349410939842764800, 6977881195084800000, 173541988447027200000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS sigma(1) = n and sigma(n) = 1. LINKS Table of n, a(n) for n=0..24. EXAMPLE In case of n = 4. ----+------------- 1 | [4, 2, 3, 1] 2 | [4, 3, 2, 1] In case of n = 5. ----+---------------- 1 | [5, 3, 2, 4, 1] 2 | [5, 3, 4, 2, 1] 3 | [5, 4, 2, 3, 1] 4 | [5, 4, 3, 2, 1] PROG (Ruby) def A(n) (2..n - 1).to_a.permutation.select{|i| (2..n - 1).all?{|j| i[j - 2] * j >= n}}.size end def A330432(n) (0..n).map{|i| A(i)} end p A330432(10) CROSSREFS Sequence in context: A301802 A318249 A093045 * A317887 A083694 A179040 Adjacent sequences: A330429 A330430 A330431 * A330433 A330434 A330435 KEYWORD nonn AUTHOR Seiichi Manyama, Mar 01 2020 EXTENSIONS a(16)-a(24) from Giovanni Resta, Mar 03 2020 STATUS approved

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Last modified September 10 16:42 EDT 2024. Contains 375791 sequences. (Running on oeis4.)