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 A181666 Odd part of a(n) is of form (4^k-1)/3. 2
 1, 2, 4, 5, 8, 10, 16, 20, 21, 32, 40, 42, 64, 80, 84, 85, 128, 160, 168, 170, 256, 320, 336, 340, 341, 512, 640, 672, 680, 682, 1024, 1280, 1344, 1360, 1364, 1365, 2048, 2560, 2688, 2720, 2728, 2730, 4096, 5120, 5376, 5440, 5456, 5460, 5461, 8192, 10240 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers that need at most one (3x+1) operation to reach 1 in the Collatz conjecture. Also, members of A023758 divisible by 3, divided by 3 (conjectured). LINKS FORMULA a(n) = (4 * 2^A055086(n-1) - 2^A082375(n-1))/3. - André Hallqvist, Jul 17 2019 MATHEMATICA Select[Range[2600], IntegerQ@ Log[4, 3 # + 1] &@ NestWhile[#/2 &, #, IntegerQ[#/2] &] &] (* Michael De Vlieger, May 12 2017 *) PROG (PARI) for(n=2, 2000, o=3*n/2^valuation(n, 2)+1; b=ispower(o); if(b&&round(sqrtn(o, b\2))==4, print1(n, ", "))) (Python) for a in range (2, 17):   print()   for b in range(a-2, -1, -2):     print(((1<

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Last modified January 20 12:15 EST 2022. Contains 350472 sequences. (Running on oeis4.)