add archive links for original fortran helanal

docs/source/getting_started.rst

@@ -82,7 +82,7 @@ Further Options
 Citations
 ---------
 
-helanal implements the HELANAL algorithm from  [Bansal2000]_, which itself
+helanal is based on the `HELANAL algorithm`_ from  [Bansal2000]_, which itself
 uses the method of [Sugeta1967]_ to characterise each local axis. Please cite 
 them when using this module in published work.
 
@@ -94,3 +94,5 @@ them when using this module in published work.
    HELANAL - A program to characterise helix geometry in proteins.
    *J Biomol Struct Dyn.*  17(5):811-819.
 
+.. _`HELANAL algorithm`:
+   https://web.archive.org/web/20090226192455/http://www.ccrnp.ncifcrf.gov/users/kumarsan/HELANAL/helanal.f

docs/source/properties_computed.rst

@@ -1,9 +1,10 @@
 Computed Properties
 ===================
 
-Helanal follows the procedure of Sugeta and Miyazawa [Sugeta1967]_. Properties
-are computed for a 'window' of four consecutive :math:`C_α` atoms, and this 
-window is then slid along the length of the helix in one-residue steps.
+Helanal follows the procedure of Sugeta and Miyazawa [Sugeta1967]_ and is based
+on the `Fortran HELANAL implementation`_ by [Bansal2000]_. 
+Properties are computed for a 'window' of four consecutive :math:`C_α` atoms, 
+and this window is then slid along the length of the helix in one-residue steps.
 
 For each window consisting of atoms :math:`c_i`, :math:`c_{i+1}`, 
 :math:`c_{i+2}`, :math:`c_{i+3}`, the vectors :math:`\mathbf{B_1}`, 
@@ -111,3 +112,6 @@ All angles are in degrees.
 
 A summary of the results, including mean, sample standard deviation and mean 
 absolute deviation is also provided in ``results.summary``.
+
+.. _`Fortran HELANAL implementation`:
+   https://web.archive.org/web/20090226192455/http://www.ccrnp.ncifcrf.gov/users/kumarsan/HELANAL/helanal.f