Does there exist a pattern for the coefficients in a negative binomial expansion? This question is not about the explicit formula, but rather if there exist a continuation for Pascal's triangle.
(a+b)−2=?(a+b)−1=?(a+b)0=1(a+b)1=1a+1b(a+b)2=1a2+2ab+1b2(a+b)3=1a3+3a2b+3ab2+1b3
It would obviously not be a triangle given that it's an infinite sum, but it seems reasonable that there should be a similar interpretation.
Answer
There is a continuation respecting the addition law
(p+1q)=(pq)+(pq−1)
This way we can write
(1+x)−3001−3x6x2−10x315x4−21x538x6…(1+x)−2001−2x3x2−4x35x4−6x57x6…(1+x)−1001−xx2−x3x4−x5x6…(1+x)0001000000(1+x)1001x00000(1+x)20012xx20000(1+x)30013x3x2x3000
For negative exponents −n with n>0 we have
(1+x)−n=∞∑j=0(−nj)xj=∞∑j=0(n+j−1j)(−1)jxj
Hint: See table 164 in Concrete Mathematics by R.L. Graham, D.E. Knuth and O. Patashnik.
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