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I am trying to apply a series expansion on a function x[t1, t2,...tn], with an expansion parameter a. For n = 2, the function is x[t1, t2], and this is my series expansion:

solRule2 = x -> (Sum[a^i Subscript[x, i][#1, #2], {i, 0, 2}] &);
x[t1, t2] /. solRule2

The result is given correctly as

Subscript[x, 0][t1, t2] + a Subscript[x, 1][t1, t2] + a^2 Subscript[x, 2][t1, t2]

This works also with derivatives, and this matters to me:

(x^(2,0))[t1,t2]/. solRule2

(Subscript[x, 0]^(2, 0))[t1, t2] + a (Subscript[x, 1]^(2,0))[t1, t2] + a^2 (Subscript[x, 2]^(2, 0))[t1, t2]

Now, I can manually extend the rule to n = 3,

solRule3 = x -> (Sum[a^i Subscript[x, i][#1, #2, #3], {i, 0, 3}] &);

but what I want to do is to extend it to the generic n. I tried this

solRuleN = x -> (Sum[a^i Subscript[x, i][##], {i, 0, Length[{##}]}] &);

This works well with the function x[t1,t2] and in general with x[t1,t2, ..., tn], but it fails with the derivatives:

(x^(1, 1))[t1, t2]/.solRuleN

gives output 0. I don't understand why this happens.

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The problem boils down to the fact that

Derivative[1][f[##] &]

0 &

Which is, in my opinion unexpected. More in: Derivative of a pure function with SlotSequence


The fix is to inject a sequence of Slots (#1, #2...) of length equal to the number of arguments our function accepts:

series[n__] := With[{
    l = Length[{n}],
    slots = Array[Slot, Length[{n}]]
  },
  Sum[a^i Subscript[x, i] @@ slots, {i, 0, l}] &
]

solRuleN = {
   x[t__] :> series[t],
   Derivative[n__][x] :> Derivative[n][series[n]]
}; 


Derivative[1, 1, 1][x][t1, t2, t3] /. solRuleN
(Subscript[x, 0]^(1,1,1))[t1,t2,t3]+a (Subscript[x, 1]^(1,1,1))[t1,t2,t3]+a^2 (Subscript[x, 2]^(1,1,1))[t1,t2,t3]+a^3 (Subscript[x, 3]^(1,1,1))[t1,t2,t3]
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  • 1
    $\begingroup$ slightly more concise: series[n__] := Length[{n}] /. l_ :> (Array[Slot, l] /. slots_ :> (Sum[a^i Subscript[x, i] @@ slots, {i, 0, l}] &)) $\endgroup$ – Mr.Wizard Jul 30 '17 at 14:16

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