Similar to 3つの立方数の和

Smale's problems

Moving sofa problem

List of unsolved problems in statistics

Navier–Stokes existence and smoothness

Burnside problem

Pierce–Birkhoff conjecture

Fermat–Catalan conjecture

Kobon triangle problem

Farrell–Jones conjecture

Hodge conjecture

Millennium Prize Problems

Yang–Mills existence and mass gap

Lychrel number

Class number problem

Ruth–Aaron pair

0.999...

Zero to the power of zero

One-dimensional space

1 + 2 + 3 + 4 + ⋯

1 + 2 + 4 + 8 + ⋯

Root of unity

Ones' complement

1 − 1 + 2 − 6 + 24 − 120 + ⋯

1 − 2 + 3 − 4 + ⋯

1 − 2 + 4 − 8 + ⋯

Plane (mathematics)