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I want to define a recursive sequence and then ask Mathematica to print a specific value:

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Am I doing something wrong?

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migrated from math.stackexchange.com May 21 '12 at 5:17

1 Answer

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First, you need to use "delayed substitution" so that Mathematica knows it needs to wait with evaluating expression until you type G[1]. Secondly, you can't have arithmetic expression on the left-hand side. so the second line should be:

G[y_]:=2G[y-1].
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Thank you very much – Stefanos May 19 '12 at 12:06
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y_ doesn't have anything to do with delayed substitution. It just means any argument (indicated by _), to be named y. If you leave away the underscore the call would work only for the literal symbol y. – Sjoerd C. de Vries May 21 '12 at 5:24
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Also it's not strictly true that you can't have an arithmetic expression on the left hand side; it just gives completely different semantics from the one intended: The original definition would have been used when calling as G[y+1] (assuming y has no value assigned at the point of call). Indeed, when using a pattern, it can even make sense to have arithmetic arguments, e.g.: G[0]:=1;G[i_Integer/;i>0]:=2*G[i-1];G[y_+n_Integer/;n>0]:=2*G[y+n-1] Then you could write G[a+4] and get the result 16*G[a]. – celtschk May 21 '12 at 8:17

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