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)+⋯+(2n2n−1)+(2n2n)
Applying the property (mk)=(mm−k) to the second half of the list of summands in RHS above:
4n=(2n0)+(2n1)+⋯+(2nn−1)+(2nn)+(2nn−1)+⋯(2n1)+(2n0)⏟(mk)=(mm−k) 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 (2n−1)=0 at the beginning just to make explicit the pattern being developed):
4n=((2n−1)+(2n0))+((2n0)+(2n1))+⋯+((2nn−1)+(2nn))
Finally, using the property (mk)+(mk−1)=(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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