Reindexing a double divisor sum: pairs `(k, j)` with `k ∣ n`, `j ∣ n/k` correspond to pairs `(m, k)` with `m ∣ n`, `k ∣ m` via `m = kj`.

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Reindexing a double divisor sum: pairs `(k, j)` with `k ∣ n`, `j ∣ n/k` correspond to pairs `(m, k)` with `m ∣ n`, `k ∣ m` via `m = kj`. · Prove2Me