A302927 Half-cumulants of a Fibonacci-geometric probability distribution.

0, 3, 11, 105, 1487, 28065, 662111, 18744705, 619117727, 23370059265, 992427775391, 46826909698305, 2430439250993567, 137613995282046465, 8441167989376455071, 557605650894240475905, 39465264036137261935007, 2979418998248159878129665

Offset

0,2

Comments

If F(k) is the k-th Fibonacci number A000045(k), then p(k)=F(k-1)/2^k is a normalized probability distribution on the positive integers.

For example, it is the probability that k coin tosses are required to get two heads in a row, or the probability that a random series of k bits has its first two consecutive 1's at the end.

The g.f. for this distribution is g(x) = x^2/(4-2x-x^2) = (1/4)x^2 + (1/8)x^3 + (1/8)x^4 + (3/32)x^5 + ....

The cumulants of this distribution, defined by the cumulant e.g.f. log(g(e^x)), appear to be integers. They form sequence A302926.

The cumulants appear to be even for n >= 0. Dividing them by 2 gives this sequence.

The n-th moments about zero of this distribution, known as raw moments, are defined by a(n) = Sum_{k>=1} (k^n)p(k). They also appear to be integers and form sequence A302922.

For n >= 1, the raw moments also appear to be even. Dividing them by 2 gives sequence A302923.

The central moments (i.e., the moments about the mean) also appear to be integers. They form sequence A302924.

For n >= 1, the central moments appear to be even. Dividing them by 2 gives sequence A302925.

Note: Another probability distribution on the positive integers that has integral moments and cumulants is the geometric distribution p(k)=1/2^k. The sequences related to these moments are A000629, A000670, A052841, and A091346.

Links

Albert Gordon Smith, Table of n, a(n) for n = 0..300

Christopher Genovese, Double Heads

Formula

E.g.f.: (1/2)*log(g(e^x)) where g(x) = x^2/(4-2*x-x^2) is the g.f. for the probability distribution.

Example

a(0)=0 is half the 0th cumulant of the distribution. The 0th cumulant is always zero.
a(1)=3 is half the 1st cumulant, which is half the mean.
a(2)=11 is half the 2nd cumulant, which is half the variance.

Mathematica

Module[{max, r, g},
  max = 17;
  r = Range[0, max];
  g[x_] := x^2/(4 - 2 x - x^2);
  (1/2) r! CoefficientList[Normal[Series[Log[g[Exp[x]]], {x, 0, max}]], x]
]

Prog

(PARI) concat(0, Vec(serlaplace(log(exp(2*x)/(4-2*exp(x)-exp(2*x))))/2)) \\ Michel Marcus, Apr 17 2018

Crossrefs

Cumulants: A302926.

Raw moments: A302922.

Raw half-moments: A302923.

Central moments: A302924.

Central half-moments: A302925.

Cf. A000045, A000629, A000670, A052841, A091346.

Sequence in context: A092245 A238446 A337734 * A105413 A241587 A183381

Adjacent sequences: A302924 A302925 A302926 * A302928 A302929 A302930

Keyword

nonn

Author

Albert Gordon Smith, Apr 15 2018

Status

approved