Gauge Fixing in xPert

[Nathan Moynihan]

Lets say I want to expand the Einstein-Hilbert Lagrangian to nth order.

What is the best of imposing a Gauge condition, i.e. the De Donder gauge? 

I know I could do it via MakeRule, but would that mean defining a rule for __every__ possible term that might be subject to the gauge condition? Is there a better way?

Many Thanks,

Nathan

[Mario Herrero Valea]

Hi,

Do you want to fix the gauge condition classically? 

Then the easiest way is to use AutomaticRules. You just need to be careful on how you write the rule, since it will look for the lhs in your formulas and change it for the rhs. In other words, if you want to implement De Donder gauge CD[-a]@h[a,b]-1/2 CD[b]@h[a,-a]=0, the easiest way is to write it like

AutomaticRules[H,MakeRule[{CD[a]@h[-a,b],1/2 CD[b]@h[a,-a]}]]

Cheers,

Mario

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[Nathan Moynihan]

Thank you Mario