triangular number
, where n is a positive integer. The first few triangular numbers are 1, 3, 6, 10, and 15, and the reason for the name can be seen from the figure.
First five triangular numbers
See also pentagonal number, tetrahedral number.
- Riemann hypothesis
- Riemannian manifold
- Riemann integral
- Riemann-Lebesgue lemma
- Riemann mapping theorem
- Riemann sphere
- Riemann surface
- Riemann zeta function
- Riemann–Roch Theorem
- Riesz representation theorem
- right angle
- right-angled triangle
- right-circular
- right derivative
- right-handed system
- right inverse
- right-regular
- rigid body
- rigid motion
- ring
- ring of integers
- rise
- risk
- Rn
- Robin boundary condition