= Massless p-form duality
{title2=$A_p\longleftrightarrow B_{D-p-2},\quad dB=*dA$}
For a free <massless p-form gauge field> with $0\le p\le D-2$, its field equation makes $*dA$ closed. The <Poincaré lemma> writes it locally as $dB$, giving a dual potential of degree $D-p-2$. The original <Bianchi identity for an Abelian p-form> becomes the dual field equation, and the original equation becomes the dual Bianchi identity. The two descriptions have equal <little group> counts $\binom{D-2}{p}$. Global fluxes and top forms require separate treatment.
// Target: quantum-field-theory.bigb
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