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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

Table of n, a(n) for n=1..51.

Index entries for sequences related to 3x+1 (or Collatz) problem

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<<a)-(1<<b))//3, end =", ")

# André Hallqvist, Jul 25 2019

CROSSREFS

Cf. A023758, A055086, A082375.

Sequence in context: A216686 A114652 A191288 * A067943 A067937 A003592

Adjacent sequences:  A181663 A181664 A181665 * A181667 A181668 A181669

KEYWORD

nonn

AUTHOR

Ralf Stephan, Nov 18 2010

STATUS

approved

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