Need to know the origin of this equation, it's used in algebra riccati equation as a solution for H-infinity

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This equation $$ R:= D_{1*}^TD_{1*} - \pmatrix{\gamma^2 I_m & 0\\0 & 0} $$ is used with algebra riccati equation in H-infinity design solution, i need to know how did we get it? This is the article: Direct Formulae for the $H_\infty$ Sub-Optimal Central Controller