This "mathematical head" sounds to me like they'd be an extraordinary person... maybe even an extraordinary man!

Having further context for the novel here helps but it's so interesting seeing the ideas of the extraordinary man being fleshed out slowly before getting to the conversation we know and love. It seems like this is the hook for an analytical essay about saving mankind. Even though the conversation is yet to happen about Rasko's ideas regarding the extraordinary man, it's easy to see how Dostoevsky it guiding the reader by planting the seed. In a funny way, it's his way of highlighting the absurd aspects of the idea before Rasko offers his own commentary on the issues. It's also key to note how right after there is a mention of a "instinctive dislike of history." This sounds like commentary from the author about the controversial views that many have on whether historical figures are truly "extraordinary" or not. When there is a later mention of Napoleon and others like Muhammad, it's clear that he is nodding towards figures that not all audiences would agree on, just not Rasko makes claims that not everyone is quick to concur with.