QUADINTERP is the jRidges module of jLab.

 QUADINTERP  Fast quadratic interpolation for arbitrary-sized arrays.
    QUADINTERP is a low-level function called by RIDGEINTERP.
    XI=QUADINTERP(T1,T2,T3,X1,X2,X3,TI) interpolates a function X
    within an independent variable T using a quadratic algorithm.
    The function values X1--X3 may be arrays of any size provided 
    they are all the same size.  The "time" values T1--T3 may be 
    scalars, or arrays of the same size as X1--X3.  
    If T1--T3 and X1--X3 are non-scalar arrays, T should either be
    a scalar or an array of the same size as the other arguments.
    If T1--T3 and X1--X3 are all scalars, T may be of any size, and
    XI will have the same size as T.
    The function values X1--X3 may be real or complex.
    QUADINTERP uses the exact algebraic expressions to operate
    looplessly on matrices, thus it is very fast.    
    Locating maxima or minima
    QUADINTERP can also be used to find the extreme value of X(T), 
    that is its maxima or minima.
    [XE,TE]=QUADINTERP(T1,T2,T3,X1,X2,X3) with only six input arguments
    solves for TE, the time at which X(T) obtains an extreme value XE.
    XE and TE are the same size as the input arrays.
    Algorithm details
    Let X and T be related as
               X = A*T.^2 + B*T + C 
    The coefficients A, B, and C are uniquely determined by three (T,X)
    pairs.  QUADINTERP solves for these coefficients and uses the 
    result to interpolate X to any other value of T.   
    Usage:   x=quadinterp(t1,t2,t3,x1,x2,x3,t);
    This is part of JLAB --- type 'help jlab' for more information
    (C) 2006 J.M. Lilly --- type 'help jlab_license' for details

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