Wheeler, Mechanics
Wheeler, Mechanics
Wheeler, Mechanics
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δS =<br />
=<br />
=<br />
∫<br />
∫<br />
∫<br />
L (x + δx, x ′ + δx ′ ) dλ −<br />
L (x, x ′ ) + ∂L ∂L<br />
δx +<br />
∂x ∂ẋ δx′ + 1 2!<br />
∂ 2 L<br />
L (x, x ′ ) dλ<br />
∂ 2 L<br />
∂x∂x (δx)2<br />
+ ∂2 L<br />
∂x∂ẋ δxδx′ + 1 2! ∂x ′ ∂x ′ (δx′ ) 2 − L (x, x ′ ) dt<br />
∫ ∂L ∂L<br />
δx +<br />
∂x ∂x ′ δx′ + 1 ∂ 2 L<br />
2! ∂x (λ) ∂x (λ) (δx)2<br />
∂ 2 L<br />
+<br />
∂x (λ) ∂x ′ (λ) δxδx′ + 1 ∂ 2 L<br />
2! ∂x ′ (λ) ∂x ′ (λ) (δx′ ) 2<br />
Now, to integrate each of the various terms by parts, we need to insert some delta functions. Look at one<br />
term at a time:<br />
∫<br />
∫<br />
∂L<br />
d ∂L<br />
∂x ′ δx′ = −<br />
dλ ∂x ′ δx<br />
I 2 = 1 ∫<br />
dλ ∂2 L<br />
2! ∂x∂x (δx)2<br />
= 1 ∫ ∫<br />
dλ 1 dλ 2 δ (λ 1 − λ 2 ) ∂2 L<br />
2<br />
∂x∂x δx (λ 1) δx (λ 2 )<br />
∫<br />
I 3 = dλ ∂2 L<br />
∂x∂x ′ δxδx′<br />
= 1 ∫ ∫<br />
dλ 1 dλ 2 δ (λ 1 − λ 2 )<br />
2<br />
and finally,<br />
× ∂2 L<br />
∂x∂x ′ (δx (λ 1) δx ′ (λ 2 ) + δx (λ 2 ) δx ′ (λ 1 ))<br />
= − 1 ∫ ∫ ( (<br />
)<br />
d<br />
dλ 1 dλ 2 δ (λ 1 − λ 2 ) ∂2 L<br />
2<br />
dλ 2 ∂x∂x ′<br />
+ d (<br />
))<br />
δ (λ 1 − λ 2 ) ∂2 L<br />
dλ 1 ∂x∂x ′ δx (λ 1 ) δx (λ 2 )<br />
= − 1 ∫ ∫ ( ∂<br />
dλ 1 dλ 2 δ (λ 1 − λ 2 ) ∂2 L<br />
2<br />
∂λ 2 ∂x∂x ′<br />
+ ∂<br />
∂λ1 δ (λ 1 − λ 2 ) ∂2 L<br />
∂x∂x ′<br />
d ∂ 2 )<br />
L<br />
+δ (λ 1 − λ 2 )<br />
dλ 1 ∂x∂x ′ δx (λ 1 ) δx (λ 2 )<br />
= − 1 ∫ ∫<br />
d ∂<br />
dλ 1 dλ 2<br />
(δ 2 )<br />
L<br />
(λ 1 − λ 2 )<br />
2<br />
dλ 1 ∂x∂x ′ δx (λ 1 ) δx (λ 2 )<br />
I 4 = 1 ∫<br />
∂ 2 L<br />
dλ<br />
2! ∂x ′ (λ) ∂x ′ (λ) (δx′ ) 2<br />
= 1 ∫ ∫<br />
∂ 2 L<br />
dλ 1 dλ 2 δ (λ 1 − λ 2 )<br />
2<br />
∂x ′ (λ 2 ) ∂x ′ (λ 1 ) δx′ (λ 1 ) δx ′ (λ 2 )<br />
= 1 ∫ ∫<br />
(<br />
∂ ∂<br />
∂ 2 )<br />
L<br />
dλ 1 dλ 2 δ (λ 1 − λ 2 )<br />
2<br />
∂λ 1 ∂λ 2 ∂x ′ (λ 1 ∂x ′ δx (λ 1 ) δx (λ 2 )<br />
(λ 2 )<br />
= 1 ∫ ∫ (<br />
∂ ∂<br />
∂ 2 )<br />
L<br />
dλ 1 dλ 2 δ (λ 1 − λ 2 )<br />
2<br />
∂λ 1 ∂λ 2 ∂x ′ ∂x ′ δx (λ 1 ) δx (λ 2 )<br />
32