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A178414 Least odd number in the Collatz (3x+1) preimage of odd numbers not a multiple of 3. 2
1, 3, 9, 7, 17, 11, 25, 15, 33, 19, 41, 23, 49, 27, 57, 31, 65, 35, 73, 39, 81, 43, 89, 47, 97, 51, 105, 55, 113, 59, 121, 63, 129, 67, 137, 71, 145, 75, 153, 79, 161, 83, 169, 87, 177, 91, 185, 95, 193, 99, 201, 103, 209, 107, 217, 111, 225, 115, 233, 119, 241, 123, 249 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

The odd non-multiples of 3 are 1, 5, 7, 11,... (A007310). The odd multiples of 3 have no odd numbers their Collatz pre-image. The next odd number in the Collatz iteration of a(2n) is 6n-1. The next odd number in the Collatz iteration of a(2n+1) is 6n+1. For each non-multiple of 3, there are an infinite number of odd numbers in its Collatz pre-image. For example:

Odd pre-images of 1: 1, 5, 21, 85, 341,... (A002450)

Odd pre-images of 5: 3, 13, 53, 213, 853,... (A072197)

Odd pre-images of 7: 9, 37, 149, 597, 2389,...

Odd pre-images of 11: 7, 29, 117, 469, 1877,...(A072261)

In each case, the pre-image sequence is t(k+1) = 4*t(k) + 1 with t(0)=a(n). The array of pre-images is in A178415.

LINKS

Matthew House, Table of n, a(n) for n = 1..10000

Index entries for linear recurrences with constant coefficients, signature (0,2,0,-1).

FORMULA

a(n) = (n - 1)*(3 - (-1)^n) + 1. [Bogart B. Strauss, Sep 20 2013, adapted to the offset by Matthew House, Feb 14 2017]

From Matthew House, Feb 14 2017: (Start)

G.f.: x*(1 + 3*x + 7*x^2 + x^3)/((1 - x)^2*(1 + x)^2).

a(n) = 2*a(n-2) - a(n-4). (End)

MATHEMATICA

Riffle[1+8*Range[0, 50], 3+4*Range[0, 50]]

CROSSREFS

Sequence in context: A146179 A294734 A214661 * A220654 A302158 A337974

Adjacent sequences:  A178411 A178412 A178413 * A178415 A178416 A178417

KEYWORD

nonn,easy

AUTHOR

T. D. Noe, May 28 2010

STATUS

approved

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Last modified December 4 15:12 EST 2020. Contains 338927 sequences. (Running on oeis4.)