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 A318354 Triangle read by rows: T(n,k) is the number of permutations p of {1..n} such that p(1)=k and p(i+1) < p(i) iff a strict majority of {1..n} \ {p(1)..p(i)} are < p(i). 1
 1, 1, 1, 2, 1, 2, 5, 3, 3, 5, 16, 11, 8, 11, 16, 62, 46, 35, 35, 46, 62, 286, 224, 178, 143, 178, 224, 286, 1519, 1233, 1009, 831, 831, 1009, 1233, 1519, 9184, 7665, 6432, 5423, 4592, 5423, 6432, 7665, 9184, 62000, 52816, 45151, 38719, 33296, 33296, 38719, 45151, 52816, 62000 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS If you shuffle n cards numbered 1 to n and then turn them over one at a time, guessing whether the next will be larger than the previous by the (optimal) rule that you guess "larger" unless there are more cards remaining smaller than the one just revealed, T(n,k) is the number of arrangements such that the first card revealed is k and you guess correctly every time. LINKS Alois P. Heinz, Rows n = 1..141, flattened The Riddler Express, O. Roeder, solution to 2018-Aug 17 problem by K. Hudson FORMULA For k <= n/2 + 1: T(n+1,k) = Sum_{i=k..n} T(n,i); For k >= n/2 + 1: T(n+1,k) = Sum_{i=1..k-1} T(n,i). T(n+1,k+1) = f(n,k), where f(n,k) is the auxiliary function defined in the formula for A144188. EXAMPLE Suppose you are playing with four cards and you initially turn over a "2". You guess "larger" because there are two larger cards, 3 and 4, remaining, and only 1 smaller card, 1, remaining. You continue playing in this way, guessing larger unless there are (strictly) more smaller cards remaining. You guess correctly every time if the order of the cards was 2,3,4,1; 2,4,3,1; or 2,4,1,3. Thus T(4,2) = 3. The triangle begins:         1        1 1       2 1 2      5 3 3 5   16 11 8 11 16 62 46 35 35 46 62 CROSSREFS T(n+1,1) = A144188(n). Sequence in context: A124218 A025165 A212431 * A106480 A099602 A151703 Adjacent sequences:  A318351 A318352 A318353 * A318355 A318356 A318357 KEYWORD nonn,tabl AUTHOR Glen Whitney, Aug 24 2018 STATUS approved

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Last modified April 19 10:38 EDT 2019. Contains 322255 sequences. (Running on oeis4.)