Rigidity and flexibility of discrete conjugate nets with flexible $3 \times 3$-subnets

Abstract

Discrete conjugate nets (also known as quad-surfaces) are polyhedral surfaces made of quadrilaterals connected in the combinatorics of the square grid. A generic discrete conjugate net is rigid. The present article contains counterexamples to an erroneous statement that a non-degenerate discrete conjugate net with flexible $3 \times 3$-subnets is flexible and proves a corrected version of it under stronger non-degeneracy assumptions.