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May 10, 2023 at 2:01 comment added Max Alekseyev A closed expression for $g(n,m)$: $$g(n,m) = (n+m+1)\binom{n+m}n \int_0^1 (1+(\alpha-1)t)^n (1+(\beta-1)t)^m {\rm d}t,$$
May 9, 2023 at 4:28 vote accept Lili Si
May 5, 2023 at 5:12 history edited Notamathematician CC BY-SA 4.0
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May 3, 2023 at 21:16 comment added Peter Mueller @Notamathematician Your answer is not "useless" as you guessed the correct form. Note that writing the OP in terms of $g(n,m)=\binom{n+m}{m}f(n,m)$ results in a simpler recursion ($\frac{\alpha n}{n+m}$ replaced with $\alpha$, and the other factor with $\beta$) which should also allow for a generating function proof.
May 3, 2023 at 8:51 comment added Notamathematician @PeterTaylor, thak you for comment! Could you write your own answer including the proof? In that case, I could delete my useless answer based on the experimental result.
May 3, 2023 at 8:46 comment added Peter Taylor The obvious way to prove it would be to show that the identity holds at the boundary conditions and is preserved by the recurrence. Since the terms can be grouped trivially by the exponents of $\alpha$ and $\beta$ this should be easy.
May 3, 2023 at 8:04 history edited Notamathematician CC BY-SA 4.0
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May 3, 2023 at 7:44 history undeleted Notamathematician
May 3, 2023 at 7:44 history edited Notamathematician CC BY-SA 4.0
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May 3, 2023 at 7:16 history deleted Notamathematician via Vote
May 3, 2023 at 6:42 history answered Notamathematician CC BY-SA 4.0