# Quantum-Mechanical Particle in an Equilateral Triangle

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The particle in an equilateral triangle is the simplest quantum-mechanical problem that has a nonseparable but exact analytic solution. The Schrödinger equation can be written with on and outside an equilateral triangle of side . The ground-state solution corresponds to an energy eigenvalue . The general solutions have the form with and , with energies . The Hamiltonian transforms under the symmetry group so eigenfunctions belong to one of the irreducible representations , or *. *The* *states labeled by quantum numbers , including the ground state , are nondegenerate with symmetry . All other integer combinations give degenerate pairs of and states. Noninteger quantum numbers belong to twofold degenerate levels.

Contributed by: Wai-Kee Li (Chinese University of Hong Kong) and S. M. Blinder (April 2008)

Open content licensed under CC BY-NC-SA

## Snapshots

## Details

Snapshot 1: contour plot of ground state

Snapshot 2: degenerate pair of , states

Snapshot 3: lowest-energy states

Reference: W.-K. Li and S. M. Blinder, "Solution of the Schrödinger Equation for a Particle in an Equilateral Triangle," *Journal of Mathematical Physics*, 26(11), 1985 pp. 2784–2786.

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