Exercise 2.5.3. Given the incidence matrix from exercise 2.5.1 and any vector in the row space of show that . Prove the same result based on the linear system . What is the implication if , , and are currents into each node?

Answer: From exercise 2.5.1 we have the incidence matrix

If is in the row space of then we have

for some set of scalar coefficients , , and so that

We then have

We therefore have for all vectors in the row space of .

Turning to the system we have

which corresponds to the system of equations

We then have

The 3 by 3 incidence matrix represents a graph with three nodes and three edges. The first row represents edge 1 from node 2 to node 1 (i.e., leaving node 2 and entering node 1). The second row represents edge 2 from node 3 to node 2. The third row represents edge 3 from node 3 to node 1.

Each node of the graph is represented by a column of and thus by a row of . If the vector represents current sources at each node ( at node 1, at node 2, and at node 3) then the fact that means that the net current into each node is zero (Kirchoff’s Current Law).

NOTE: This continues a series of posts containing worked out exercises from the (out of print) book Linear Algebra and Its Applications, Third Edition by Gilbert Strang.

If you find these posts useful I encourage you to also check out the more current Linear Algebra and Its Applications, Fourth Edition, Dr Strang’s introductory textbook Introduction to Linear Algebra, Fourth Edition and the accompanying free online course, and Dr Strang’s other books.