patternMinor
Solution of complex recurrence relation
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Problem
Do anyone have any idea how to solve this recurrence?$$T(k) = \sum_{i=1}^{k/2} {k \choose i}T(i)T(k-i)$$
I am working with this kind of recurrence for the first time and don't have much idea of how to proceed. But, for beginning I was trying to guess the solution and use substitution method. But, in the guessing part itself I am stuck. It seems like the solution should be exponential to $k$.
I am working with this kind of recurrence for the first time and don't have much idea of how to proceed. But, for beginning I was trying to guess the solution and use substitution method. But, in the guessing part itself I am stuck. It seems like the solution should be exponential to $k$.
Solution
Everything becomes much simpler if when $n$ is odd we stop the sum at $\lfloor n/2 \rfloor$, and when $k$ is even we discount the term corresponding to $k/2$ by a half. In that case you can write
$$ 2T(k) = \sum_{i=1}^{k-1} \binom{k}{i} T(i) T(k-i) $$
and so, putting $T(0) = 0$, we get that for $k \geq 2$,
$$ 2T(k) = \sum_{i=0}^k \binom{k}{i} T(i) T(k-i). \tag{1}$$
(When $k = 1$ this reads $2T(1) = 0$.)
Now define the exponential generating function
$$ P = \sum_{n \geq 0} T(n) \frac{x^n}{n!}. $$
Put $T(1) = c$. The identity (1) implies the equation
$$ 2(P-cx) = P^2. $$
Solving the quadratic, we obtain
$$ P = 1 - \sqrt{1 - 2cx} = 1 - \sum_{n \geq 0} \frac{(2n)!}{(1-2n)(n!)^24^n} (2cx)^n = \sum_{n \geq 1} \frac{(2n)!(c/2)^n}{(2n-1)n!} \frac{x^n}{n!}. $$
We conclude that for $n \geq 1$,
$$
T(n) = \frac{(2n)!(c/2)^n}{(2n-1)n!} \sim \frac{1}{\sqrt{2} n} \left( \frac{2c}{e} n \right)^n.
$$
The first few terms (starting with $T(1)$) are
$$
c,c^2,3c^3,15c^4,105c^5,\ldots
$$
$$ 2T(k) = \sum_{i=1}^{k-1} \binom{k}{i} T(i) T(k-i) $$
and so, putting $T(0) = 0$, we get that for $k \geq 2$,
$$ 2T(k) = \sum_{i=0}^k \binom{k}{i} T(i) T(k-i). \tag{1}$$
(When $k = 1$ this reads $2T(1) = 0$.)
Now define the exponential generating function
$$ P = \sum_{n \geq 0} T(n) \frac{x^n}{n!}. $$
Put $T(1) = c$. The identity (1) implies the equation
$$ 2(P-cx) = P^2. $$
Solving the quadratic, we obtain
$$ P = 1 - \sqrt{1 - 2cx} = 1 - \sum_{n \geq 0} \frac{(2n)!}{(1-2n)(n!)^24^n} (2cx)^n = \sum_{n \geq 1} \frac{(2n)!(c/2)^n}{(2n-1)n!} \frac{x^n}{n!}. $$
We conclude that for $n \geq 1$,
$$
T(n) = \frac{(2n)!(c/2)^n}{(2n-1)n!} \sim \frac{1}{\sqrt{2} n} \left( \frac{2c}{e} n \right)^n.
$$
The first few terms (starting with $T(1)$) are
$$
c,c^2,3c^3,15c^4,105c^5,\ldots
$$
Context
StackExchange Computer Science Q#24569, answer score: 7
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