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PHYS08200604018 Shamik Banerjee - Homi Bhabha National ...

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4.5. EXAMPLES 55<br />

Let us determine the symmetries of this partition function. For this it will be useful to work<br />

in terms of the original unscaled variables (ˇρ, ˇσ, ˇv) and at the end go back to the rescaled<br />

variables. The first term on the right hand side of (4.5.2.6) has the usual Sp(2, Z) symmetries<br />

acting on the variables (ˇρ, ˇσ, ⎛ˇv). However not⎞<br />

all of these are symmetries of the second term.<br />

a 1 b 1 c 1 d 1<br />

Given an Sp(2, Z) matrix ⎜ a 2 b 2 c 2 d 2<br />

⎟<br />

⎝ a 3 b 3 c 3 d 3<br />

⎠ , it is a symmetry of the second term provided<br />

a 4 b 4 c 4 d 4<br />

⎛<br />

⎞<br />

a ′ 1 b ′ 1 c ′ 1 d ′ 1<br />

its action on (ˇρ, ˇσ, ˇv) can be regarded as an Sp(2, Z) action ⎜ a ′ 2 b ′ 2 c ′ 2 d ′ 2 ⎟<br />

⎝ a ′ 3 b ′ 3 c ′ 3 d ′ ⎠ on (ˇρ, ˇσ+ 1, ˇv) 2<br />

3<br />

a ′ 4 b ′ 4 c ′ 4 d ′ 4<br />

followed by a translation on ˇσ by 1/2. Since ⎛ a translation of ⎞ ˇσ by 1/2 can be regarded as a<br />

1 0 0 0<br />

symplectic transformation with the matrix ⎜ 0 1 0 1/2<br />

⎟<br />

⎝ 0 0 1 0 ⎠ , the above condition takes the<br />

0 0 0 1<br />

form:<br />

⎛<br />

⎞ ⎛<br />

⎞ ⎛<br />

⎞ ⎛<br />

⎞<br />

1 0 0 0 a 1 b 1 c 1 d 1 a ′ 1 b ′ 1 c ′ 1 d ′ 1 1 0 0 0<br />

⎜ 0 1 0 1/2<br />

⎟ ⎜ a 2 b 2 c 2 d 2<br />

⎟<br />

⎝ 0 0 1 0 ⎠ ⎝ a 3 b 3 c 3 d 3<br />

⎠ = ⎜ a ′ 2 b ′ 2 c ′ 2 d ′ 2 ⎟ ⎜ 0 1 0 1/2<br />

⎟<br />

⎝ a ′ 3 b ′ 3 c ′ 3 d ′ ⎠ ⎝<br />

3 0 0 1 0 ⎠ . (4.5.2.8)<br />

0 0 0 1 a 4 b 4 c 4 d 4 a ′ 4 b ′ 4 c ′ 4 d ′ 4 0 0 0 1<br />

This gives<br />

⎛<br />

⎞ ⎛<br />

a ′ 1 b ′ 1 c ′ 1 d ′ 1 a 1 b 1 c 1 d 1 − 1 ⎜ a ′ 2 b ′ 2 c ′ 2 d ′ 2 ⎟<br />

⎝ a ′ 3 b ′ 3 c ′ 3 d ′ ⎠ = b 2 1<br />

⎜ a 2 + 1<br />

⎝<br />

a 2 4 b 2 + 1b 2 4 c 2 + 1c 2 4 d 2 + 1(d 2 4 − b 2 ) − 1b 4 4<br />

3 a 3 b 3 c 3 d 3 − 1b a ′ 4 b ′ 4 c ′ 4 d ′ 2 3<br />

4 a 4 b 4 c 4 d 4 − 1b 2 4<br />

⎞<br />

⎟<br />

⎠ . (4.5.2.9)<br />

The coefficients a i , b i , c i and d i are integers. Requiring that there exist integer a ′ i, b ′ i, c ′ i and d ′ i<br />

satisfying the above constraints we get further conditions on a i , b i , c i and d i . These take the<br />

following form:<br />

a 4 , b 4 , c 4 , b 1 , b 3 ∈ 2 Z, b 4 − 2(d 4 − b 2 ) ∈ 4 Z . (4.5.2.10)<br />

⎛<br />

a 1 b 1 c 1 d 1<br />

On the other hand the requirement that the original matrix ⎜ a 2 b 2 c 2 d 2<br />

⎟<br />

⎝ a 3 b 3 c 3 d 3<br />

⎠ is symplectic,<br />

a 4 b 4 c 4 d 4<br />

⎞<br />

together with the first set of conditions given in (4.5.2.10), can be used to show that b 2 and d 4<br />

are both odd. As a result (b 2 − d 4 ) is even, and hence b 4 must be a multiple of 4 in order to<br />

satisfy (4.5.2.10). Thus we have<br />

a 4 = 2â 4 , b 4 = 4̂b 4 , c 4 = 2ĉ 4 , b 1 = 2̂b 1 , b 3 = 2̂b 3 , â 4 ,̂b 4 , ĉ 4 ,̂b 1 ,̂b 3 ∈ Z . (4.5.2.11)

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