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Problems > Geometric Series Modulo > Editorial

Geometric Series Modulo — Solution & Editorial

Back to the Problem

Closed form A(qN−1)/(q−1) using the modular inverse of q−1 — but q = 1 needs the special case A·N.

Complexity: O(log N)

Watch out: q = 1 divides by zero in the closed form — handle it as A·N separately.