Bell #s Generalized: e^e^..^(e^x-1)
Leroy Quet
qqquet at mindspring.com
Fri Jul 25 03:39:04 CEST 2003
It seems like I have heard of this before. And the idea is obvious.
But the next-highest-order sequence above the standard Bell numbers is
not in the on-line Encyclopedia of Integer Sequences, IF I have
calculated the first few terms correctly (by hand).
It is known that, if B(k) is the k_th (standard) Bell number,
sum{k=0 to oo} B(k) x^k /k! =
exp(exp(x)-1).
But if we want, for a fixed m >= 2, {B(m,k)}, where
sum{k=0 to oo} B(m,k) x^k /k! =
exp(exp(...exp(exp(x)-1)..)),
with m 'exp's,
(therefore, B(2,k) is a standard Bell #)
then:
B(1,k) = 1 for all k >= 0 (violating the generating-function, due to
B(1,0) = 1, and not 0);
B(m,0) = 1;
and:
B(m,j+1) =
sum{k=0 to j} binomial(j,k) B(m-1,k) B(m,j-k)
(If I have not erred)
By the way, {B(3,k)} to start:
1, 1, 2, 6, 23, 106,...
Am I even right about any of this?
(If not, this would explain why {B(3,k)} is not in the EIS.)
Thanks,
Leroy Quet
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