Mass Matrix, Damping Matrix, and Stiffness Matrix

M \frac{d^{2} u}{d t^{2}} + C \frac{d u}{d t} + K (u) u = f

M	Concentrated (diagonal) mass matrix
C	Diagonal damping matrix C = αM + βdiag[K₁₁(u),K₂₂(u),...,K_nn(u)]
K	Stiffness matrix
f	Vector of external forces
U	Discretized displacement vector

Do you have any questions?