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<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>Λ</mml:mi><mml:mi>CDM</mml:mi></mml:math>universe in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>f</mml:mi><mml:mo stretchy="false">(</mml:mo><mml:mi>R</mml:mi><mml:mo stretchy="false">)</mml:mo></mml:math>gravity
Several different explicit reconstructions of $f(R)$ gravity are obtained from the background Friedmann-La\^{\i}matre-Robertson-Walker expansion history. It is shown that the only theory whose Lagrangian is a simple function of the Ricci scalar $R$, that admits an exact $\ensuremath{\Lambda}\mathrm{CDM}$ expansion history, is standard general relativity with a positive cosmological constant and …