OFFSET
0,2
COMMENTS
The ternary Cantor measure, defined many ways, is the unique Borel measure mu on the unit interval [0,1] satisfying the following recurrence relation for any measurable set E: mu(E) = mu(phi_0(E))/2 + mu(phi_2(E))/2. Here, for j in {0,1,2}, phi_j: [0,1] to [0,1] is the linear function which sends x in [0,1] to (x+j)/3. For any nonnegative integer k, we define the k-th moment to be I(k) to be the integral of x^k with respect to mu. The described sequence I(0), I(1), I(2), ... is rational and this sequence a(0), a(1), a(2), ... is the sequence of denominators of I(0), I(1), I(2), ....
For the purpose of computing I(k), we note the following recurrence relation: I(0) = 1 and for all positive k, I(k) = (1/(3^k-1))*((1/2) * Sum_{j=0..k-1} binomial(k, j) + (1/2) * Sum_{j=0..k-1} binomial(k, j) * 2^(k-j) * I(j)).
More generally, for any N-dimensional nonnegative vector alpha = (alpha_0, ..., alpha_{N-1}) whose entries sum to 1, there exists a unique Borel measure mu = mu^{alpha} on [0,1] so that for any measurable set E, the following identity holds: mu(E) = Sum_{k=0..N-1} alpha_k * mu(phi_k(E)). Here, for j in {0, 1, ..., N-1}, phi_j: [0,1] to [0,1] is the linear function which sends x in [0,1] to (x+j)/N. Defining I(k) to be the integral of x^k with respect to mu, the following recurrence relation holds: I(0) = 1 and for all positive k, I(k) = (1/(N^k-1)) * Sum_{n=0..N-1} alpha_n * Sum_{j=0..k-1} binomial(k, j) * n^(k-j)*I(j).
LINKS
Michael De Vlieger, Table of n, a(n) for n = 0..97
David H. Bailey, Jonathan M. Borwein, Richard E. Crandall, and Michael G. Rose, Expectations on fractal sets, Applied Mathematics and Computation, Vol. 220 (2013), pp. 695-721, alternative link.
Steven N. Harding, Alexander W. N. Riasanovsky, Moments of the weighted Cantor measures, arXiv:1908.05358 [math.FA], 2019.
Math Stack Exchange, Integrating f(x) = x for x in C, the Cantor set, with respect to a certain measure
MATHEMATICA
a[0] = 1; a[n_] := a[n] = Sum[Binomial[n, j]*2^(n - j - 1)*a[j], {j, 0, n - 1}]/(3^n - 1); Table[Denominator[a[i]], {i, 0, 19}] (* Amiram Eldar, Aug 03 2019 *)
PROG
(Sage)
def am(m, alpha):
N = len(alpha)
am = [1]
for a in [1..m]:
mm = 0
for k in [0..N-1]:
for r in [0..a-1]:
mm += alpha[k]*binomial(a, r)*k^(a-r)*am[r]
mm /= (N^a-1)
am.append(mm)
return am
[p.denominator() for p in am(15, (1/2, 0, 1/2))]
CROSSREFS
KEYWORD
nonn,frac
AUTHOR
Alexander Riasanovsky, Jun 10 2019
EXTENSIONS
More terms from Amiram Eldar, Aug 03 2019
STATUS
approved