In combinatorics, Sun's curious identity is the following identity involving binomial coefficients, first established by Zhi-Wei Sun in 2002:
\( (x+m+1)\sum_{i=0}^m(-1)^i\dbinom{x+y+i}{m-i}\dbinom{y+2i}{i} -\sum_{i=0}^{m}\dbinom{x+i}{m-i}(-4)^i=(x-m)\dbinom{x}{m}. \)
Proofs
After Sun's publication of this identity, five other proofs were obtained by various mathematicians:
Panholzer and Prodinger's proof via generating functions;
Merlini and Sprugnoli's proof using Riordan arrays;
Ekhad and Mohammed's proof by the WZ method;
Chu and Claudio's proof with the help of Jensen's formula;
Callan's combinatorial proof involving dominos and colorings.
References
Callan, D. (2004), "A combinatorial proof of Sun's 'curious' identity" (PDF), INTEGERS: The Electronic Journal of Combinatorial Number Theory, 4: A05, arXiv:math.CO/0401216.
Chu, W.; Claudio, L.V.D. (2003), "Jensen proof of a curious binomial identity" (PDF), INTEGERS: The Electronic Journal of Combinatorial Number Theory, 3: A20.
Ekhad, S. B.; Mohammed, M. (2003), "A WZ proof of a 'curious' identity" (PDF), INTEGERS: The Electronic Journal of Combinatorial Number Theory, 3: A06.
Merlini, D.; Sprugnoli, R. (2002), "A Riordan array proof of a curious identity" (PDF), INTEGERS: The Electronic Journal of Combinatorial Number Theory, 2: A08.
Panholzer, A.; Prodinger, H. (2002), "A generating functions proof of a curious identity" (PDF), INTEGERS: The Electronic Journal of Combinatorial Number Theory, 2: A06.
Sun, Zhi-Wei (2002), "A curious identity involving binomial coefficients" (PDF), INTEGERS: The Electronic Journal of Combinatorial Number Theory, 2: A04.
Sun, Zhi-Wei (2008), "On sums of binomial coefficients and their applications", Discrete Mathematics, 308 (18): 4231–4245, arXiv:math.NT/0404385, doi:10.1016/j.disc.2007.08.046.
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