which museum exhibits the portrait of sigismondo pandolfo malatesta by piero della francesca ?
select distinct var_uri where brack_open dbr_Portrait_of_Sigismondo_Pandolfo_Malatesta dbo_museum var_uri sep_dot dbr_Portrait_of_Sigismondo_Pandolfo_Malatesta dbo_author dbr_Piero_della_Francescabrack_close