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MIT ODE class assignment here

http://math.mit.edu/classes/18.03/spr10/pset_04.pdf

talks about solving $\dot{x} + 2x = e^{3t}\cos(4t)$ and using ERF to do it. I know how to solve this using integrating factors, but what is ERF? The solution is below, but explicit steps are not mentioned.

http://math.mit.edu/classes/18.03/spr10/pset_04_sol.pdf

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It seems to mean exponential response formula. See math.mit.edu/classes/18.03/spr10's LTI operators and exponential signals (subsection 10.2 of Operators and the exponential response formula).

Quote:

The Exponential Response Formula ties together many different parts of this course. Since the most important signals are exponential, and the most important differential operators are LTI operators, this single formula solves most of the ODEs you are likely to face in your future.