1st derivatives of f wrt delta and tau
function f3deltatau extends Modelica.Icons.Function; input SI.Density d "density"; input SI.Temperature T "temperature (K)"; output Real delta "dimensionless density"; output Real tau "dimensionless temperature"; output Real fdelta "dimensionless dervative of Helmholtz function wrt delta"; output Real ftau "dimensionless dervative of Helmholtz function wrt tau"; end f3deltatau;