The use of 'equation' in this sentence

[Tom]

Hi

Could you please explain to me the use of 'equation' in this sentence?

[Mister Micawber]

.
Equation = 'A complex of variable elements or factors'..
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Native English teacher at Mister Micawber's

[Tom]

Many thanks, MM.

But I need some more light here. Could you please explain the sentence a bit?

Tom

[Yankee]

Hi Tom

I'd say it means that various characteristics of the friendship had changed -- more of one aspect, less of another, for example.
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"Nearly all men can stand adversity, but if you want to test a man's character, give him power." ~ Abraham Lincoln