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@MartinSleziak While reading some research papers I am facing doubt like from equation 2 how it derives equation 3 ,but I have no idea how we arrived at 3 so how to show my try then? so if i ask it in the main it would be like straight away asking questions in the main without trying,what should i do? any help?

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still let me state where i am facing problem, its this paper "Master Stability Functions for Synchronized Coupled Systems Louis M. Pecora and Thomas L. Carroll" , eqn(1) to eqn(3)

how he obtained those variational equations?