PERIODINDEX is the jRidges module of jLab.

 PERIODINDEX  Returns time index in increments of instantaneous period.
 
    INDEX=PERIODINDEX(DT,OMEGA,N) returns a time index INDEX that skips 
    every N times the instantaneous period 2*pi/OMEGA.
 
    OMEGA is a column vector of instantaneous frequency in radians per unit 
    time as computed by INSTMOM. DT is the sample time, a scalar.  The
    units of 2*pi/OMEGA should be the same as the units of DT.
 
    Then PERIODINDEX constructs an index into a vector of the same length
    as OMEGA that skips every N times the instantaneous period 2*pi/OMEGA.
    Thus N=1 returns an index with one sample per period, etc.
 
    The first and last N periods are omitted from the index.  Specifically,
    the first value of INDEX is N times first value of 2*pi/OMEGA, and 
    the last value of INDEX is not larger than the length of OMEGA minus
    N times the last value of 2*pi/OMEGA.
 
    If NaNs are found within the OMEGA, this is interpreted as being
    separate ridges output by RIDGEWALK.  Then INDEX will be a cell array 
    with the period index for each ridge of OMEGA in a separate cell.
 
    INDEX=PERIODINDEX(OMEGA,N) also works, in which case DT is taken to be
    as unity.  Thus OMEGA must have units of radians per sample interval.
 
    PERIODINDEX is useful with ELLIPSEPLOT for plotting ellipses a
    specified number of periods apart.
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    Cell array input / output
 
    PERIODINDEX returns cell array output given cell array input.  
 
    That is, if OMEGA is a cell arrays of length K, containing K different 
    numerical arrays, then INDEX will also be cell arrays of length K.  
 
    In this case DT may be either a scalar, or an array of length K.
 
    If the cells of OMEGA are found to contain NaNs, these are interpreted
    as separating individual ridges, and INDEX will in this case be a cell
    array of length OMEGA with each element containing one cell per ridge.
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    See also ELLIPSEPLOT.
 
    Usage: index=periodindex(dt,omega,N);
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    This is part of JLAB --- type 'help jlab' for more information
    (C) 2011--2016 J.M. Lilly --- type 'help jlab_license' for details

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