Presidents of Brazil

Actually, brazilian presidents are not always known solely by their last names. Dilma Rousseff is more commonly known as Dilma. Lula is how everyone calls her predecessor, whose predecessor, by the way, is almost always referred to as FHC. Juscelino Kubistschek is known as JK and Tancredo Neves is often just Tancredo. This must come from the fact that Brazilians like to refer to everyone by their first name rather than their family name, even if it's someone in a position of power. Students at Brazilian universities rarely call their professors by their last name.

Some of the earliest presidents, however, were already known by more than one last name, such as Deodoro da Fonseca (whose first name was actually Manuel), and Rodrigues Alves (whose first name was Francisco). It wouldn't therefore make sense to accept anything but both the names for which they are known.