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9.2 Black Holes Once Again

355

Curvature of the Extremal Spaces In order to facilitate the discussion of the various solutions found by means of the σ -model method, it is useful to consider the general form of the Riemann tensor associated with the metrics (9.2.7) in the extremal case. To this effect we introduce the vielbein 1-forms:

E0 = exp U dt

2

E1

= exp

U

 

τ 2

2

(9.2.30)

E2

= exp

U

1

 

τ

2

E3 = exp

U

1

 

 

τ sin[θ ]

 

2

 

and the corresponding spin connection:

 

 

 

dEa + ωab Ecηbc = 0

(9.2.31)

Defining the curvature 2-form in the standard way:

 

Rab = ab + ωac ωdbηcd

(9.2.32)

we find that it is diagonal:

 

 

 

 

 

R01 = C1E0 E1

 

R02 = C2E0 E2

 

R03 = C2E0

E3

(9.2.33)

R12

= C3E1

E2

 

R13

= C3E1

E3

 

R23

= C4E3

E4

 

and involves four independent differential expressions in the function

C1(τ ) = − 1 eU (τ )τ 3 τ U (τ )2 + 2U (τ ) + τ U (τ )

4

C2(τ ) = 1 eU (τ )τ 3U (τ ) τ U (τ ) + 2

8

C3(τ ) = 1 eU (τ )τ 3 U (τ ) + τ U (τ )

4

C4(τ ) = − 1 eU (τ )τ 3U (τ ) τ U (τ ) + 4

8

U (τ ), namely

(9.2.34)

We will consider the behavior of these four independent component of the Riemann tensor in various solutions that we present some pages later.

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