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I investigate the Birch and Swinnerton-Dyer conjecture, which ties 
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together the constellation of invariants attached to an abelian variety.  
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I attempt to verify this conjecture for certain specific modular abelian 
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varieties of dimension greater than one.  The key idea is to use 
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Barry Mazur's notion of visibility, coupled with explicit computations, 
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to produce lower bounds on the Shafarevich-Tate group.  I have
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not finished the 
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proof of the conjecture in these examples; this would 
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require computing explicit upper bounds on the order of this group.
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I next describe how to compute in 
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spaces of modular forms of weight at least two.
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I give an integrated package for computing, in many
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cases, the following invariants of a modular abelian variety: the
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modular degree, the rational part of the special value of the
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-function, the order of the component group at primes of
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multiplicative reduction, the period lattice, upper and lower bounds
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on the torsion subgroup, and the real volume.  Taken together, these
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algorithms are frequently sufficient to compute the odd part of the
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conjectural order of the Shafarevich-Tate group of an analytic
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rank  optimal quotient of , with  square-free.  I have
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not determined the exact structure of the component
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group, the order of the component group at primes whose square divides
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the level, or the exact order of the torsion subgroup in all cases.
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However, I do provide generalizations of some of the above algorithms
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to higher weight forms with nontrivial character.
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