The Basel problem — what it asks, and how it was solved
For almost ninety years nobody could say what 1 + 1/4 + 1/9 + ⋯ adds up to. How Euler found the answer, π²/6, how he proved it, and three later proofs that each find the π somewhere else.
For almost ninety years nobody could say what 1 + 1/4 + 1/9 + ⋯ adds up to. How Euler found the answer, π²/6, how he proved it, and three later proofs that each find the π somewhere else.