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 A259542 a(1) = 1, for n > 1 a(n) = smallest number not already in the sequence such that the arithmetic mean of two neighboring terms is a triangular number. 7
 1, 5, 7, 13, 17, 3, 9, 11, 19, 23, 33, 39, 51, 21, 35, 37, 53, 57, 15, 27, 29, 43, 47, 25, 31, 41, 49, 61, 71, 85, 97, 59, 73, 83, 99, 111, 45, 65, 67, 89, 93, 63, 69, 87, 95, 115, 125, 147, 159, 81, 75, 107, 103, 79, 77, 55, 101, 109, 131, 141, 165, 177 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS A259604(n) = (a(n) + a(n+1)) / 2; conjecture: sequence is a permutation of the odd numbers, see also A259260, A259429; a(A259543(n)) = 2*n-1. LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 PROG (Haskell) import Data.List (delete) a259542 n = a259542_list !! (n-1) a259542_list = 1 : f 1 [3, 5 ..] where    f x zs = g zs where      g (y:ys) = if a010054 ((x + y) `div` 2) == 1                    then y : f y (delete y zs) else g ys CROSSREFS Cf. A259260, A259429, A010054, A005408, A259604. Sequence in context: A064465 A314319 A314320 * A236204 A152810 A260405 Adjacent sequences:  A259539 A259540 A259541 * A259543 A259544 A259545 KEYWORD nonn AUTHOR Reinhard Zumkeller, Jun 30 2015 STATUS approved

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Last modified September 17 11:03 EDT 2019. Contains 327129 sequences. (Running on oeis4.)