Data analysis

Explanation of tables
*green Function exists in octave 2.0.x
Noted incompatibilities and missing features
+yellow Function exists in matcompat
Author and home for the function.
?orange Function doesn't exist
Explanation of what you can do instead.
xred Function doesn't exist, and no work-around

Sorting
*sort
+sortrows
Daniel Calvelo

Filtering
*filter
+filter2
Paul Kienzle
*conv
+conv2
Andy Adler; alternatively, Ariel Tankus' Octave Image Processing Toolbox contains a less compatible version.
xconvn
*deconv
*detrend

Fourier analysis
*fft
*fft2
xfftn
*ifft
*ifft2
xifftn
*fftshift
+ifftshift
Paul Kienzle

Interpolation
+interp1interpolation for arbitrarily spaced data
Paul Kienzle; relies on lookup to find the correct interval
+interpftFourier interpolation for equally spaced data
Paul Kienzle
+interp22D interpolation
Kai Habel
xinterp3
xinterpn
+spline
Paul Kienzle, based on Kai Habel's csapi, but doesn't matrix xi; Netlib?? has pppack in Fortran which could be the basis of an Octave spline toolbox
+ppvalevaluate piece-wise polynomial
+mkppcreate piece-wise polynomial structure
+unmkppextract piece-wise polynomial structure
Paul Kienzle

Geometry
The geometry toolbox from Kai Habel provides delaunay, tsearch, convhull, voronoi, griddata
?delaunay
xdsearch
?tsearch
?convhull
?voronoi
?griddata
?inpolygon
octave-sources, but with caveats.
xrectint
+polyarea
David M. Doolin

Polynomials
*roots
*poly
*polyval
*polyvalm
*residue
Note Octave is missing [b a]=residue(r,p,k), but resid2wp.m on the matworks archive defines it.
*polyfit
+polyder
extended by Paul Kienzle and others to handle a*b and a/b