# A polynomial recurrence involving partial derivatives

Define recursively polynomials $f_n(a,b)$ by $$f_0(a,b)=1,\ \ f_n(0,b)=0\ \mathrm{for}\ n>0$$ $$\frac{\partial}{\partial a}f_n(a,b) = f_{n-1}(b-a,1-a).$$ For instance, $$f_1(a,b) = a,\ \ f_2(a,b) = \frac 12(2ab-a^2)$$ $$f_3(a,b) = \frac 16(a^3-3a^2-3ab^2+6ab).$$ Is there a nice'' solution to this recurrence, e.g., a formula for the generating function $\sum_{n\geq 0}f_n(a,b)x^n/n!$? What I am really interested in is $f_n(1,1)$. For the motivation, see the solution to Exercise~4.56(d) (pg. 645) of Enumerative Combinatorics, vol.1, 2nd ed.

• Hi Richard, I added the page number of the solution; I hope you don't mind. – Suvrit Feb 7 '12 at 22:20
• Here are the values of $f_n(1,1)$ for $0\le n\le 15$: $1,1,1,1,2,5,14,47,182,786,3774,19974,115236,720038,4846512,34950929$ (of course they don't mach anything in the OEIS). – Pietro Majer Feb 8 '12 at 0:25
• You mean $n!f_n(1,1)$. – Richard Stanley Feb 8 '12 at 1:07
• (yes sorry, I meant the numerators) – Pietro Majer Feb 8 '12 at 9:01
• It is in the OEIS: A096402, submitted by a certain Stanley ;). – Brendan McKay Feb 8 '12 at 10:43

There seems to be a PDE for $g(a,b,x)=\sum_{n\ge0}f_n(a,b)x^n$, which can be thought of as a boundary value problem in the triangle $0\lt a\lt b\lt1$. $$g_{aab}+g_{abb}+x^3g=0$$ ($x$ is a parameter and subscripts are derivatives) with boundary values $g(0,b,x)=1$, $g_a(a,a,x) = x$, and $g_{ab}(a,1,x) = x^2$. This comes from iterating the $f_n$ recurrence, after Pietro's remarks that $(a,b)\to(b-a,1-a)$ has period 3 suggested looking at third derivatives. Does that determine $g$ uniquely, nicely? I don't know yet. [Edit: I wrongly wrote $g$ at first using $\frac{x^n}{n!}$.]