( Period ( ( is ( ( the reciprocal ) ( of ( the frequency ) ) ) ) . ) )
(ROOT (S (NP (NN Period)) (VP (VBZ is) (NP (NP (DT the) (NN reciprocal)) (PP (IN of) (NP (DT the) (NN frequency))))) (. .)))
Period is the reciprocal of the frequency.
(ROOT (S (NP (NP (DT The) (NN frequency)) (PP (IN of) (NP (DT a) (NN wave)))) (VP (VBZ is) (NP (NP (DT the) (NN reciprocal)) (PP (IN of) (NP (NP (DT the) (NN period)) (PP (IN of) (NP (DT the) (NN wave))))))) (. .)))