# Sum of Indexed Suprema

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## Theorem

Let $\left \langle {a_i} \right \rangle_{i \mathop \in I}$ be a family of elements of the real numbers $\R$ indexed by $I$.

Let $\left \langle {b_j} \right \rangle_{j \mathop \in J}$ be a family of elements of the real numbers $\R$ indexed by $J$.

Let $R \left({i}\right)$ and $S \left({j}\right)$ be propositional functions of $i \in I$, $j \in J$.

Let $\displaystyle \sup_{R \left({i}\right)} a_i$ and $\displaystyle \sup_{S \left({j}\right)} b_j$ be the indexed suprema on $\left \langle {a_i} \right \rangle$ and $\left \langle {b_j} \right \rangle$ respectively.

Then:

- $\displaystyle \left({\sup_{R \left({i}\right)} a_i}\right) + \left({\sup_{S \left({j}\right)} b_j}\right) = \sup_{R \left({i}\right)} \left({\sup_{S \left({j}\right)} \left({a_i + b_j}\right)}\right)$

## Proof

\(\displaystyle \left({\sup_{R \left({i}\right)} a_i}\right) + \left({\sup_{S \left({j}\right)} b_j}\right)\) | \(=\) | \(\displaystyle \sup_{R \left({i}\right)} \left({a_i + \sup_{S \left({j}\right)} b_j}\right)\) | Sum with Maximum is Maximum of Sum | ||||||||||

\(\displaystyle \) | \(=\) | \(\displaystyle \sup_{R \left({i}\right)} \left({\sup_{S \left({j}\right)} \left({a_i + b_j}\right)}\right)\) | Sum with Maximum is Maximum of Sum |

$\blacksquare$

## Sources

- 1997: Donald E. Knuth:
*The Art of Computer Programming: Volume 1: Fundamental Algorithms*(3rd ed.) ... (previous) ... (next): $\S 1.2.3$: Sums and Products: Exercise $35$