We present in detail a calculation of the spectral dimension for a class of fractal trees called NTD (i.e., ``nice trees of dimension D,'' defined as trees whose branches are splitting in r every time the distance from the origin is doubled, where r is an integer greater than 1) which presents nonanomalous diffusion. This is performed by an analytical technique, related to an exact rescaling of the time variable, which can be extended to more general geometrical structures.
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