Pattern sample

data = {{

x_{1}, R_{1}

},{

x_{2}, R_{2}

},{

x_{3}, R_{3}

}...,{

x_{m}, R_{m}

}};
loglog=Function[{x,y},{Log[x/

{[C a^{2 +}]}_{0}

],Log[y]}];
datalog=Apply[loglog,data,{1}];

peakr = Function[{

A, B, C, x

},Piecewise[{{

A

+(1/2)*Log[1-(2/3)*

B x

]
+

C

*(1-(1-(2/3)*

B x

)^(3/2),

x

<3/2/

B

,Infinity}}]];

fit = NonlinearModelFit[datalog,{peakr[

A, B, C, x

],{

A > 0, B > 0, C > 0

}},
{{

A, A_{0}

},{

B, B_{0}

},{

C, C_{0}

}},x,Method->NMinimize];
fit['ParameterTable']