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Summary form only given. We derive an exact expression for the electromagnetic mode density, and hence the group velocity, for a finite N period, one-dimensional photonic band-gap structure. We begin by deriving a general formula for the mode density in terms of the complex transmission coefficient of an arbitrary index profile. Then we develop a formula that gives the N-period mode density in terms of the transmission coefficient of the unit cell. The special cases of mode-density enhancement and suppression at the photonic band edge and at mid gap, respectively are derived. The specific example of a quarter-wave stack is analyzed, and applications to 3D structures, spontaneous emission control, delay lines, band-edge lasers, and superluminal tunneling times are discussed.