[m.jalali]

I am working with udf in ansys fluent . I need an udf for viscosity Changes with pH . Viscosity as a function of error function varies with pH(erf(pH)). I wrote an udf is as follows:
#include "udf.h"
#include "mem.h"
//#include "math.h"
DEFINE_PROPERTY(cell_viscosity,c,t)
{
real c_mu_eff ;
real mu_lam_0= 0.001 ; /* base viscosity (acid viscosity at pH=0 or 7)*/
real mu_lam_m= 200. ; /* maximum increment in the viscosity w.r.t*/
real C_ca; /* mass freaction of calcium*/
real pH;
real C_max_ca= 10.; /* corresponding calcium ion where viscosity reaches the maximom value (wt)*/
real mw=36.5; /* molcolar whight of hcl*/
real w2=1.1, w3=2.; /* constant coefficiet*/
C_ca= C_YI(c,t,1)*100.; /* mass freaction of calcium ion*/
pH=-log10(C_YI(c,t,2)*C_R(c,t)/mw);

c_mu_eff = mu_lam_0*(1.+mu_lam_m*exp(-(C_ca-C_max_ca)/w2)*(erf(PH-1)+1.)/w3);
C_UDMI(c,t,4)= mu_lam_0*(1.+mu_lam_m*exp(-(C_ca-C_max_ca)/w2)*(erf(PH-1)+1.)/w3);

return c_mu_eff;
}

But when I compile the code gives the error as follows:
error: erf: undeclared variable
Can anyone help me؟

[pakk]

Remove the two /'s before #include "math.h".