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 A231610 The least k such that the Collatz (3x+1) iteration of k contains 2^n as the largest power of 2. 1
 1, 2, 4, 8, 3, 32, 21, 128, 75, 512, 151, 2048, 1365, 8192, 5461, 32768, 14563, 131072, 87381, 524288, 184111, 2097152, 932067, 8388608, 5592405, 33554432, 13256071, 134217728, 26512143, 536870912, 357913941, 2147483648, 1431655765, 8589934592, 3817748707 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Very similar to A225124, where 2^n is the largest number in the Collatz iteration of A225124(n). The only difference appears to be a(8), which is 75 here and 85 in A225124. The Collatz iteration of 75 is {75, 226, 113, 340, 170, 85, 256, 128, 64, 32, 16, 8, 4, 2, 1}. LINKS FORMULA a(n) = 2^n for odd n. EXAMPLE The iteration for 21 is {21, 64, 32, 16, 8, 4, 2, 1}, which shows that 64 = 2^6 is a term. However, 32 is not the first power of two. We have to wait until the iteration for 32, which is {32, 16, 8, 4, 2, 1}, to see 32 = 2^5 as the first power of two. MATHEMATICA Collatz[n_?OddQ] := 3*n + 1; Collatz[n_?EvenQ] := n/2; nn = 21; t = Table[-1, {nn}]; n = 0; cnt = 0; While[cnt < nn, n++; q = Log[2, NestWhile[Collatz, n, Not[IntegerQ[Log[2, #]]] &]]; If[q < nn && t[[q + 1]] == -1, t[[q + 1]] = n; cnt++]]; t CROSSREFS Cf. A010120, A054646 (similar sequences). Cf. A135282, A232503 (largest power of 2 in the Collatz iteration of n). Cf. A225124. Sequence in context: A243065 A289271 A223699 * A225124 A059884 A191561 Adjacent sequences:  A231607 A231608 A231609 * A231611 A231612 A231613 KEYWORD nonn AUTHOR T. D. Noe, Dec 02 2013 STATUS approved

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Last modified December 5 16:12 EST 2019. Contains 329753 sequences. (Running on oeis4.)