function pt = gsw_pt_from_CT(SA,CT)

% gsw_pt_from_CT        potential temperature from Conservative Temperature
%==========================================================================
%
% USAGE:  
%  pt = gsw_pt_from_CT(SA,CT)
%
% DESCRIPTION:
%  Calculates potential temperature (with a reference sea pressure of
%  zero dbar) from Conservative Temperature.  This function uses 1.5 
%  iterations through a modified Newton-Raphson (N-R) iterative solution 
%  proceedure, starting from a rational-function-based initial condition 
%  for both pt and dCT_dpt. 
%
% INPUT:
%  SA   =   Absolute Salinity                                      [ g/kg ]
%  CT   =   Conservative Temperature                              [ deg C ]
%
%  SA & CT need to have the same dimensions.
%
% OUTPUT:
%  pt   =   potential temperature referenced to a sea pressure 
%           of zero dbar (ITS-90)                                 [ deg C ]
%
% AUTHOR: 
%  Trevor McDougall, David Jackett, Claire Roberts-Thomson and Paul Barker. 
%          [ help_gsw@csiro.au ]
%
% VERSION NUMBER: 2.0 (26th August, 2010)
%
% REFERENCES:
%  IOC, SCOR and IAPSO, 2010: The international thermodynamic equation of 
%   seawater - 2010: Calculation and use of thermodynamic properties.  
%   Intergovernmental Oceanographic Commission, Manuals and Guides No. 56,
%   UNESCO (English), 196 pp.  Available from http://www.TEOS-10.org
%    See sections 3.1 and 3.3 of this TEOS-10 Manual.
%
%  McDougall T. J., D. R. Jackett, P. M. Barker, C. Roberts-Thomson, R.
%   Feistel and R. W. Hallberg, 2010:  A computationally efficient 25-term 
%   expression for the density of seawater in terms of Conservative 
%   Temperature, and related properties of seawater.  To be submitted 
%   to Ocean Science Discussions. 
%
%  The software is available from http://www.TEOS-10.org
%
%==========================================================================

%--------------------------------------------------------------------------
% Check variables and resize if necessary
%--------------------------------------------------------------------------

if ~(nargin == 2)
    error('gsw_pt_from_CT: Requires two inputs')
end

[ms,ns] = size(SA);
[mt,nt] = size(CT);

if (mt ~= ms | nt ~= ns)
    error('gsw_pt_from_CT: SA and CT must have same dimensions')
end

if ms == 1
    SA = SA';
    CT = CT';
    transposed = 1;
else
    transposed = 0;
end

%--------------------------------------------------------------------------
% Start of the calculation
%--------------------------------------------------------------------------

% These few lines ensure that SA is non-negative.
[I_neg_SA] = find(SA < 0);
if ~isempty(I_neg_SA)
    SA(I_neg_SA) = 0;
end

cp0 = 3991.86795711963;           % from Eqn. (3.3.3) of IOC et al. (2010).

s1 = SA*35./35.16504; 

a0 = -1.446013646344788d-2;     
a1 = -3.305308995852924d-3;     
a2 =  1.062415929128982d-4;     
a3 =  9.477566673794488d-1;    
a4 =  2.166591947736613d-3;
a5 =  3.828842955039902d-3;

b0 =  1.000000000000000d+0;
b1 =  6.506097115635800d-4;
b2 =  3.830289486850898d-3;
b3 =  1.247811760368034d-6;

a5CT = a5*CT; 
b3CT = b3*CT;
CT_factor = (a3 + a4*s1 + a5CT);
pt_num = a0 + s1.*(a1 + a2*s1) + CT.*CT_factor;
pt_den = b0 + b1*s1 + CT.*(b2 + b3CT);
pt = (pt_num)./(pt_den);

dCT_dpt = (pt_den)./(CT_factor + a5CT - (b2 + b3CT + b3CT).*pt);

% start the 1.5 iterations through the modified Newton-Rapshon iterative method. 

CT_diff = gsw_CT_from_pt(SA,pt) - CT;
pt_old = pt;
pt = pt_old - (CT_diff)./dCT_dpt; % 1/2-way through the 1st modified N-R loop
ptm = 0.5d0.*(pt + pt_old);

% This routine calls gibbs_pt0_pt0(SA,pt0) to get the second derivative 
% of the Gibbs function with respect to temperature at zero sea pressure.  

dCT_dpt = -(ptm + 273.15).*gsw_gibbs_pt0_pt0(SA,ptm)./cp0;
pt = pt_old - (CT_diff)./dCT_dpt; % end of 1st full modified N-R iteration
CT_diff = gsw_CT_from_pt(SA,pt) - CT;
pt_old = pt;
pt = pt_old - (CT_diff)./dCT_dpt; % 1.5 iterations of the modified N-R method

if transposed
    pt = pt';
end

% abs max error of result is 1.42e-14 deg C

end
