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 A239123 a(n) = 128*n - 107 for n >= 1. Third column of triangle A238475. 3
 21, 149, 277, 405, 533, 661, 789, 917, 1045, 1173, 1301, 1429, 1557, 1685, 1813, 1941, 2069, 2197, 2325, 2453, 2581, 2709, 2837, 2965, 3093, 3221, 3349, 3477, 3605, 3733, 3861, 3989, 4117, 4245, 4373, 4501, 4629, 4757, 4885, 5013, 5141 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS This sequence gives all start numbers a(n) (sorted increasingly) of Collatz sequences of length 8 following the pattern ud^6 with u (for `up'), mapping an odd number m to 3*m+1, and d (for `down'), mapping an even number m to m/2. The last entry of this Collatz sequence is required to be odd, and it is given by 6*n - 5. This appears in Example 2.1. for x = 6 in the M. Trümper paper given as a link below. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 Wolfdieter Lang, On Collatz' Words, Sequences, and Trees, J. of Integer Sequences, Vol. 17 (2014), Article 14.11.7. Manfred Trümper, The Collatz Problem in the Light of an Infinite Free Semigroup, Chinese Journal of Mathematics, Vol. 2014, Article ID 756917, 21 pages. FORMULA O.g.f.: x*(21+107*x)/(1-x)^2. EXAMPLE a(1) = 21 because the Collatz sequence of length 8 is [21, 64, 32, 16, 8, 4, 2, 1] ending in 6*1-5 = 1, and 21 is the smallest positive number following this pattern udddddd ending in an odd number. a(2) = 149 with the length 8 Collatz sequence [149, 448, 224, 112, 56, 28, 14, 7] ending in 6*2 - 5 = 7, and 149 is the second smallest start number following this pattern ud^6, ending in an odd number. MATHEMATICA CoefficientList[Series[(21 + 107 x)/(1 - x)^2, {x, 0, 50}], x] (* Vincenzo Librandi, Mar 12 2014 *) CROSSREFS Cf. A238475, A238477 (second column). Sequence in context: A259493 A253459 A041850 * A241697 A239569 A219599 Adjacent sequences:  A239120 A239121 A239122 * A239124 A239125 A239126 KEYWORD nonn,easy AUTHOR Wolfdieter Lang, Mar 10 2014 STATUS approved

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Last modified June 15 19:35 EDT 2019. Contains 324144 sequences. (Running on oeis4.)