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Putnam
2020 Putnam
A2
Putnam 2020 A2
Putnam 2020 A2
Source: 81st William Lowell Putnam Competition
February 22, 2021
Putnam
Putnam 2020
college contests
Binomial identites
Problem Statement
Let
k
k
k
be a nonnegative integer. Evaluate
∑
j
=
0
k
2
k
−
j
(
k
+
j
j
)
.
\sum_{j=0}^k 2^{k-j} \binom{k+j}{j}.
j
=
0
∑
k
2
k
−
j
(
j
k
+
j
)
.
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