Monday, 25 December 2017

Continuing "Pascal's triangle" for negative binomial exponents




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)+(pq1)




This way we can write

(1+x)30013x6x210x315x421x538x6(1+x)20012x3x24x35x46x57x6(1+x)1001xx2x3x4x5x6(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+j1j)(1)jxj




Hint: See table 164 in Concrete Mathematics by R.L. Graham, D.E. Knuth and O. Patashnik.




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