I was trying to solve a system of differential equations, and this worked:

`vector = {f1[t],f2[t],f3[t]} equations = Flatten[Table[{D[vector[[i]], t] == vector[[i]]((matrix. vector)[[i]] - vector.PayoffMatrix.vector), (vector[[i]] /. t -> 0) == vectorT0[[i]]}, {i, Length[vector]}]] NDSolve[equations, vector, {t, 0, 30}] `

But if I add "." explicitly after `vector[[i]]`

—it doesn’t:

`vector = {f1[t],f2[t],f3[t]} equations = Flatten[Table[{D[vector[[i]], t] == vector[[i]].((matrix. vector)[[i]] - vector.PayoffMatrix.vector), (vector[[i]] /. t -> 0) == vectorT0[[i]]}, {i, Length[vector]}]] NDSolve[equations, vector, {t, 0, 30}] NDSolve::ndnum: Encountered non-numerical value for a derivative at t == 0.`. `

What’s wrong here? (Mathematica 12.2)