feynlag.invariance.gauge_variation

feynlag.invariance.gauge_variation(term, fields, group)

O(α) variation coefficients of term under group, per generator.

Returns a list (length = number of generators) of expressions; all must vanish for invariance.

Each generator’s coefficient is computed from an independent single-α transformation (the α-vector is zero except for the generator under test). This is mathematically identical to expanding the full multi-α variation and picking out one α — the O(α²) cross-terms between generators vanish when the other α’s are set to zero anyway — but it keeps every intermediate expression small: only one generator matrix enters at a time. That matters for large / radical-valued representations (a general SU(N) irrep built in the Gelfand–Tsetlin basis), where the all-generators-at-once expansion of a covariant-derivative kinetic term is intractable.