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answered 12 years ago

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  1. It is not really a sage question, so I'll pass (let's just say that it's a theta series, so there's a quadratic form hidden).
  2. You just need Θ(q)=a+bq+O(q2), because then in terms of basis given by sage, it will be a times the first plus b times the second.
  3. More generally, sage gives basis in a form that makes it pretty easy to read a conjectural linear combination from the first terms of the expansion ; if you already know you got the expansion from a modular forms in the right space, that will be enough to know the equality.
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Add something about find_in_space
  1. It is not really a sage question, so I'll pass (let's just say that it's a theta series, so there's a quadratic form hidden).
  2. You just need Θ(q)=a+bq+O(q2), because then in terms of basis given by sage, it will be a times the first plus b times the second.
  3. More generally, sage gives basis in a form that makes it pretty easy to read a conjectural linear combination from the first terms of the expansion ; if you already know you got the expansion from a modular forms in the right space, that will be enough to know the equality.

EDIT: you might be interested by the method find_in_space from the modular spaces objects.