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A simple question:

I have this equation:

eq1=Derivative[0, 1][T1][x, t] - Derivative[1, 0][T0][x, t]^2 - 
 T0[x, t]*Derivative[2, 0][T0][x, t] - Derivative[2, 0][T1][x, t] == 0;

I want only to select terms that contain T0 or its derivatives only, that is:

-Derivative[1, 0][T0][x, t]^2 - T0[x, t]*Derivative[2, 0][T0][x, t]

Thanks in anticipation.

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    $\begingroup$ DeleteCases[eq1 /. Equal -> Subtract, _?(FreeQ[#,T0]&]? $\endgroup$ – Michael E2 Jan 15 '20 at 22:59
  • $\begingroup$ @MichaelE2 there is a parenthesis missing after the & symbol. Great answer though. More elegant than mine! $\endgroup$ – DiSp0sablE_H3r0 Jan 15 '20 at 23:01
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    $\begingroup$ Yeah, I'm using Gedanken Mathematica, you know the one that's basically a keyboard without a kernel. Hard to catch those typos. $\endgroup$ – Michael E2 Jan 15 '20 at 23:09
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While the structural operation

DeleteCases[eq1 /. Equal -> Subtract, _?(FreeQ[#,T0]&)]

works, I prefer using an algebraic approach on a algebraic problem.

vars = Select[Not@*FreeQ[T0]]@Variables[eq1 /. Equal -> Subtract];
coeffs = CoefficientArrays[eq1 /. Equal -> Subtract, vars];
Fold[#2 + #1.vars &, Reverse@ReplacePart[coeffs, 1 -> 0]]

Mathematica graphics

The structural approach relies on the equation being in a particular form, a flat sum of terms, which does not always happend. The algebraic approach does not. However, CoefficientArrays does rely on it being a polynomial in the variables vars.

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Block[{T1, Equal = Plus}, SetAttributes[T1, Constant]; eq1]

-Derivative[1, 0][T0][x, t]^2 - T0[x, t]*Derivative[2, 0][T0][x, t]

Or (thanks to Mr. Wizard here)

eq1 /. {s_Symbol /; StringMatchQ[SymbolName[Unevaluated@s], "T" ~~ Except["0"]] -> 0, Equal -> Plus}

-Derivative[1, 0][T0][x, t]^2 - T0[x, t]*Derivative[2, 0][T0][x, t]
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Not the most elegant solution, but you can use the Collectcommand in the following manner

eq1 =  Derivative[0, 1][T1][x, t] - 
       Derivative[1, 0][T0][x, t]^2 - 
       T0[x, t]*Derivative[2, 0][T0][x, t] - 
       Derivative[2, 0][T1][x, t]; 
       (Coefficient[#1, {T0[x, t], Derivative[1, 0][T0][x, 
       t], Derivative[1, 0][T0][x, t]^2}] & )[eq1]

This is telling you the coefficient in front of T0[x, t] and so on.

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