# Template:Math/EQ TRApproximation

 ${\displaystyle \Phi _{\mathrm {TR} }(\varpi ,z)}$ ${\displaystyle =}$ ${\displaystyle -{\biggl [}{\frac {2GM}{\pi }}{\biggr ]}{\frac {K(k)}{\sqrt {(\varpi +a)^{2}+z^{2}}}}}$ ${\displaystyle \mathrm {where:} ~~~k\equiv \{4\varpi a/[(\varpi +a)^{2}+z^{2}]\}^{1/2}}$