R<sub>0</sub>)</span> has to be satisfied in order to have a solution. The parameter <span class="nowrap">R<sub>1</sub></span> is tuned to satisfy the condition. If it's impossible to find an <span class="nowrap">R<sub>1</sub></span> in the acceptable range. Another set of parameters A, U and T have to be chosen. Suppose an acceptable <span class="nowrap">R<sub>1</sub></span> is found. Equation (4) can be written as</p><p><span class="nowrap">mt - na</span> = <span class="nowrap">S<sub>1</sub></span>,&nbsp;