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PHYS08200604018 Shamik Banerjee - Homi Bhabha National ...

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56 CHAPTER 4. GENERALITIES OF QUARTER BPS DYON PARTITION FUNCTION<br />

This determines the subgroup of Sp(2, Z) which leaves the individual terms in (4.5.2.6) invariant.<br />

To this we must add the additional element corresponding to ˇσ → ˇσ + 1 which exchanges<br />

2<br />

the two terms in (4.5.2.6). This corresponds to the symplectic transformation<br />

⎛<br />

⎞<br />

1 0 0 0<br />

⎜ 0 1 0 1/2<br />

⎟<br />

⎝ 0 0 1 0 ⎠ . (4.5.2.12)<br />

0 0 0 1<br />

The full symmetry group is then generated by the matrices:<br />

⎛<br />

⎞ ⎛<br />

⎞<br />

a 1 2̂b 1 c 1 d 1<br />

1 0 0 0<br />

⎜ a 2 b 2 c 2 d 2<br />

⎟<br />

⎝ a 3 2̂b 3 c 3 d 3<br />

⎠ and ⎜ 0 1 0 1/2<br />

⎟<br />

⎝ 0 0 1 0 ⎠ . (4.5.2.13)<br />

2â 4 4̂b 4 2ĉ 4 d 4 0 0 0 1<br />

We can easily determine how these transformations act on the rescaled variables (ˇρ, ˇσ s , ˇv s ).<br />

This is done with the help of conjugation by the symplectic matrix<br />

⎛<br />

⎞<br />

1 0 0 0<br />

⎜ 0 2 0 0<br />

⎟<br />

⎝ 0 0 1 0 ⎠ (4.5.2.14)<br />

0 0 0 1/2<br />

relating (ˇρ, ˇσ, ˇv) to (ˇρ, ˇσ s , ˇv s ). This converts the generators given in (4.5.2.13) to<br />

⎛<br />

⎞ ⎛<br />

⎞<br />

a 1<br />

̂b1 c 1 2d 1<br />

1 0 0 0<br />

⎜ 2a 2 b 2 2c 2 4d 2<br />

⎟<br />

⎝ a 3<br />

̂b3 c 3 2d 3<br />

⎠ and ⎜ 0 1 0 2<br />

⎟<br />

⎝ 0 0 1 0 ⎠ . (4.5.2.15)<br />

â 4<br />

̂b4 ĉ 4 d 4 0 0 0 1<br />

We now note that all the matrices appearing in (4.5.2.15) have the form<br />

⎛<br />

⎞<br />

∗ ∗ ∗ 0<br />

⎜ 0 ∗ 0 0<br />

⎟<br />

⎝ ∗ ∗ ∗ 0 ⎠ mod 2 , (4.5.2.16)<br />

∗ ∗ ∗ ∗<br />

with ∗ denoting an arbitrary integer subject to the condition that (4.5.2.16) describes a symplectic<br />

matrix. Furthetmore the set of matrices (4.5.2.16) are closed under matrix multiplication.<br />

Thus the group generated by the matrices (4.5.2.15) is contained in the group Ǧ<br />

consisting of Sp(2, Z) matrices of the form (4.5.2.16). It is in fact easy to show that the group<br />

generated by the matrices (4.5.2.15) is the whole of Ǧ, ı.e. any element of Ǧ given in (4.5.2.16)<br />

can be written as a product of the elements given in (4.5.2.15).

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