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A225570 The greedy smallest infinite reverse Collatz (3x+1) sequence. 2
1, 2, 4, 8, 16, 5, 10, 20, 40, 13, 26, 52, 17, 34, 11, 22, 7, 14, 28, 56, 112, 37, 74, 148, 49, 98, 196, 65, 130, 43, 86, 172, 344, 688, 229, 458, 916, 305, 610, 203, 406, 812, 1624, 541, 1082, 2164, 721, 1442, 2884, 961, 1922, 3844, 7688, 15376, 5125, 10250 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For each a(n) (where n > 4), a(n) = (a(n-1) - 1)/3 if the result is an odd integer not divisible by 3. Otherwise a(n) = 2 * a(n-1).

Going backwards from any term a(n) to a(1), this is the Collatz sequence for a(n).  Furthermore, each term in the sequence is the smallest possible term (ignoring multiples of 3) with this property given the previous term.

Multiples of 3 are ignored because after visiting a multiple of 3, subsequent terms can only double.

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

MAPLE

A225570 := proc(n)

    local a;

    option remember;

    if n <= 4  then

        2^(n-1) ;

    else

        a := (procname(n-1)-1)/3 ;

        if type(a, 'integer') and type(a, 'odd') and modp(a, 3) <> 0 then

            return a;

        else

            return procname(n-1)*2 ;

        end if;

    end if;

end proc: # R. J. Mathar, Aug 03 2013

MATHEMATICA

last = 8; Join[{1, 2, 4, 8}, Table[test = (last - 1)/3; If[OddQ[last] || ! IntegerQ[test] || IntegerQ[test/3], last = 2*last, last = (last - 1)/3]; last, {96}]] (* T. D. Noe, Aug 11 2013 *)

CROSSREFS

Cf. A014682, A070165.

Sequence in context: A276070 A070337 A269305 * A178170 A127824 A088975

Adjacent sequences:  A225567 A225568 A225569 * A225571 A225572 A225573

KEYWORD

nonn

AUTHOR

David Spies, Jul 29 2013

STATUS

approved

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Last modified September 17 03:42 EDT 2021. Contains 347478 sequences. (Running on oeis4.)