N=18  ; Nc = 36
ISOSTATIC É
just enough contacts Nc  to fix all N particles
i.e. 2N contacts     (in 2 dimensions)
not quite jammed:  remove 1 contact
Éliberates 1 FREE MODEÉe g
Free modes are É extended (like acoustic modes), heterogeneous (unlike acoustic).
Can 0-frequency free modes explain low-frequency modes of jammed system?
D. Head (2003)
L. Silbert (2005)
Nearly isostatic packings have free modes