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 A331499 T(n,k) is the number of nonintersecting arch configurations with n arches on a horizontal axis that have a starting parent arch configuration of k arches. 1
 1, 1, 1, 2, 1, 2, 4, 3, 2, 5, 10, 6, 7, 5, 14, 24, 18, 15, 19, 14, 42, 66, 42, 48, 43, 56, 42, 132, 174, 130, 116, 143, 132, 174, 132, 429, 504, 332, 374, 358, 451, 423, 561, 429, 1430, 1406, 1048, 974, 1182, 1163, 1475, 1397, 1859, 1430, 4862 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS A parent arch configuration has exactly one exterior arch and can be derived from an initial arch configuration with more than one exterior arch by using the algorithm for decreasing arches explained below. When concentric arches are placed below the different arch configurations, the number of components (loops formed) in each configuration will remain the same as the number components in that arch configuration's parent arch. Algorithm: To determine the parent arch configuration, start with all the nonintersecting arch configurations with n arches and more than one exterior arch. For each starting individual arch configuration, reduce the number of arches until there is exactly one exterior arch. Join the start of the first arch with the end of the last arch. This will eliminate one arch and shrink the number of arches from n to n-1. Continue this process until there is one exterior arch. /\ n=5 / \ /\ n=5 / /\\ /\ / \ --> /\//\//\\\ <-- --> //\\//\/\\ <-- /\ --> /\/\//\\ <-- --> /\/\/\/\ <-- /\ /\ --> //\\/\ <-- / \ --> /\/\ <-- //\/\\ parent arch k = 3 /\ parent arch k = 1. LINKS Table of n, a(n) for n=1..55. FORMULA The sum of row n = A000108(n). T(n,1) = A000682(n). T(n,n) = A000108(n-1). T(n,n-1) = A000108(n-2). T(n,n-2) = A000108(n-2) + A000108(n-3) (conjectured). EXAMPLE Triangle T(n,k) begins: n\k 1 2 3 4 5 6 7 8 9 10 11 1: 1 2: 1 1 3: 2 1 2 4: 4 3 2 5 5: 10 6 7 5 14 6: 24 18 15 19 14 42 7: 66 42 48 43 56 42 132 8: 174 130 116 143 132 174 132 429 9: 504 332 374 358 451 423 561 429 1430 10: 1406 1048 974 1182 1163 1475 1397 1859 1430 4862 11: 4210 2836 3166 3152 3906 3897 4950 4719 6292 4862 16796 CROSSREFS Cf. A000108, A000682. Sequence in context: A331136 A099312 A117505 * A209128 A209131 A165053 Adjacent sequences: A331496 A331497 A331498 * A331500 A331501 A331502 KEYWORD nonn,tabl AUTHOR Roger Ford, Feb 16 2020 STATUS approved

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Last modified May 28 01:34 EDT 2024. Contains 372900 sequences. (Running on oeis4.)