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A119921 Number of rationals in [0, 1) having at most n preperiodic bits, then at most n periodic bits. 0
2, 12, 72, 336, 1632, 6720, 29568, 120576, 499200, 2012160, 8214528, 32894976, 132882432, 532070400, 2136637440, 8551464960, 34282536960, 137135652864, 549148164096, 2196721631232, 8791208755200, 35166005231616 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Table of n, a(n) for n=1..22.

FORMULA

a(n) = 2^n * sum_{j=1..n} sum_{d|j} (2^d - 1) * mu(j/d)

EXAMPLE

The binary expansion of 7/24 = 0.010(01)... has 3 preperiodic bits (to the right of the binary point) followed by 2 periodic (i.e., repeating) bits, while 1/2 = 0.1(0)... has one bit of each type. The preperiodic and periodic parts are both chosen to be as short as possible.

a(2) = |{ 0/1 = 0.(0)..., 1/3 = 0.(01)..., 2/3 = 0.(10)..., 1/2 = 0.1(0)..., 1/6 = 0.0(01)..., 5/6 = 0.1(10)..., 1/4 = 0.01(0)..., 3/4 = 0.11(0)..., 1/12 = 0.00(01)..., 5/12 = 0.01(10)..., 7/12 = 0.10(01)..., 11/12 = 0.11(10)...}| = 12

MATHEMATICA

Table[2^n Sum[Plus@@((2^Divisors[j]-1)MoebiusMu[j/Divisors[j]]), {j, 1, n}], {n, 1, 22}]

CROSSREFS

Elementwise product of 2^n (offset 1) and A119917. Also, diagonal of A119919.

Sequence in context: A002630 A009552 A002670 * A322276 A279154 A167747

Adjacent sequences:  A119918 A119919 A119920 * A119922 A119923 A119924

KEYWORD

nonn,base,easy

AUTHOR

Brad Chalfan (brad(AT)chalfan.net), May 28, 2006

STATUS

approved

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Last modified July 30 10:27 EDT 2021. Contains 346359 sequences. (Running on oeis4.)