Tuesday, 7 January 2020

summation - Prove the identity binom2n+10+binom2n+11+cdots+binom2n+1n=4n



I've worked out a proof, but I was wondering about alternate, possibly more elegant ways to prove the statement. This is my (hopefully correct) proof:



Starting from the identity 2m=mk=0(mk) (easily derived from the binomial theorem), with m=2n:




22n=4n=(2n0)+(2n1)++(2n2n1)+(2n2n)



Applying the property (mk)=(mmk) to the second half of the list of summands in RHS above:



4n=(2n0)+(2n1)++(2nn1)+(2nn)+(2nn1)+(2n1)+(2n0)(mk)=(mmk) has been applied



Rearranging the above sum by alternately taking terms from the front and end of the summand list in RHS above (and introducing the term (2n1)=0 at the beginning just to make explicit the pattern being developed):



4n=((2n1)+(2n0))+((2n0)+(2n1))++((2nn1)+(2nn))




Finally, using the property (mk)+(mk1)=(m+1k) on the paired summands, we get the desired result:



4n=(2n+10)+(2n+11)++(2n+1n)


Answer



Why not just
22n+1=(2n+10)++(2n+1n)+(2n+1n+1)++(2n+12n+1)=(2n+10)++(2n+1n)+(2n+1n)++(2n+10)=2[(2n+10)++(2n+1n)]
Then divide each extremity by 2.



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