Control Systems
The graph of an electrical network has N nodes and B branches. The number of links, L with respect to the choice of a tree, is given by

 B + N.
  B - N + 1.
 N - B + 1.
 N - 2B - 1.

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Control Systems
A cascade of three linear time invariant systems is causal and unstable. From this we conclude that

 at least one system is unstable and at least one system is causal.
 at least one system is causal and all systems are unstable.
 the majority are unstable and the majority are causal.
 each system in the cascade is individually caused and unstable.

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