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 A322281 Number of permutations sigma such that |sigma(i+j)-sigma(i)| >= 4 for 1 <= i <= n - j, 1 <= j <= 3. 3
 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 74, 2424, 93424, 4394386, 201355480 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,17 COMMENTS 2 | a(n) for n > 1. LINKS EXAMPLE a(16) = 2: [4,8,12,16,3,7,11,15,2,6,10,14,1,5,9,13] and its reverse. PROG (Ruby) def check(d, a, i)   return true if i == 0   j = 1   d_max = [i, d - 1].min   while (a[i] - a[i - j]).abs >= d && j < d_max     j += 1   end   (a[i] - a[i - j]).abs >= d end def solve(d, len, a = [])   b = []   if a.size == len     b << a   else     (1..len).each{|m|       s = a.size       if s == 0 || (s > 0 && !a.include?(m))         if check(d, a + [m], s)           b += solve(d, len, a + [m])         end       end     }   end   b end def A322281(n)   (0..n).map{|i| solve(4, i).size} end p A322281(18) CROSSREFS Cf. A179957, A279214. Sequence in context: A083018 A240174 A295172 * A282878 A062552 A116172 Adjacent sequences:  A322278 A322279 A322280 * A322282 A322283 A322284 KEYWORD nonn,more AUTHOR Seiichi Manyama, Dec 01 2018 EXTENSIONS a(21) from Alois P. Heinz, Dec 02 2018 STATUS approved

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Last modified September 20 21:32 EDT 2021. Contains 347591 sequences. (Running on oeis4.)