# Evaluating terms in series

I'm trying to evaluate a simple expression:

Subscript[λ, n] = (2 n - 1) π/(2 L);


And then sum it up:

Sum[Subscript[λ, n], {n, 1, 3}]


The result is:

π/(2 L) + Subscript[λ, 2] + Subscript[λ, 3]


How can i get Mathematica to write the second and third term in the same way as the first?

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Do not understand the question, but may be you wanted this? lam[n_] := (2 n - 1) Pi/(2 L); Sum[ lam[n], {n, 1, 3}] –  Nasser Dec 1 '13 at 10:33
Yes that would work, but would it be possible to write each term separately? Like Pi/2L + 3Pi/2L + 5Pi/2L? –  Sami Dec 1 '13 at 11:16

I believe you are trying to limit evaluation in the sum. You can use Defer to accomplish this. I Apply (@@) it to a list for partial evaluation.

Subscript[λ, n_] := Defer @@ {(2 n - 1) Pi/2 L};

Sum[Subscript[λ, n], {n, 1, 3}]

(L π)/2 + (3 L π)/2 + (5 L π)/2


Or perhaps more to your liking:

Subscript[λ, n_] := Defer @@ {(2 n - 1) Pi/Defer[2 L]};

Sum[Subscript[λ, n], {n, 1, 3}]

π/(2 L) + (3 π)/(2 L) + (5 π)/(2 L)


By the way, if you are going to use subscripts for function definitions please read Can we use letter with a subscript as a variable in Mathematica?. A number of users would tell you to avoid them for simplicity.

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This was what I was looking for! Thanks! –  Sami Dec 1 '13 at 11:48