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A090129 Smallest exponent such that -1+3^a[n] is divisible by 2^n. +0
8
1, 2, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16384, 32768, 65536, 131072, 262144 (list; graph; listen)
OFFSET

1,2

COMMENT

A152046=1,1,1,3,5,11,=1,A001045(n+1)?. a(n+1)=A152046(n)+A152406(n+1)=2*A011782. A131577 and A011782 are companions, A131577+A011782=2^n=A000079, (and differences each other). [From Paul Curtz (bpcrtz(AT)free.fr), Jan 18 2009]

EXAMPLE

a[1]=1 since -1+3=2 is divisible by 2^1;

a[2]=a[3]=2 since -1+9=8 is divisible by 4=2^2 and also by 8=2^3;

a[5]=8 since -1+6561=6560=32.205 is divisible by 2^5.

MATHEMATICA

t=Table[Part[Flatten[FactorInteger[ -1+3^(n)]], 2], {n, 1, 130}] Table[Min[Flatten[Position[t, j]]], {j, 1, 10}]

CROSSREFS

Cf. A069895, A091512, A088660, A090739, A090740.

Sequence in context: A054243 A005864 A112433 this_sequence A001137 A123593 A122748

Adjacent sequences: A090126 A090127 A090128 this_sequence A090130 A090131 A090132

KEYWORD

nonn

AUTHOR

Labos E. and R. Stephan ((labos(AT)ana.sote.hu) and (ralf(AT)ark.in-berlin.de)), Jan 19 2004

EXTENSIONS

a(11) through a(20) by R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Aug 08 2008

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Last modified December 10 00:48 EST 2009. Contains 170565 sequences.


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