A handwritten notebook in which Britain's Enigma machine genius Alan Turing admits he is baffled by an equation could fetch up to $1million (£690,000). The 56-page book bought in Cambridge contains Turing's thoughts on the clearly tricky 'Leibniz notation dx/dy'. 'I find [it] extremely difficult to understand in spite of it having been the one I understood the best once!' he wrote. Scroll down for video . A handwritten notebook in which Britain's Enigma machine genius Alan Turing admits he is baffled by an equation could fetch up to $1million (£690,000). Above, he wrote that the Leibniz notation was 'extremely difficult' It was written at the Bletchley Park code-breaking headquarters in 1942 and paved the way for computer science. It is believed to be the only extensive Turing manuscript known to exist, the auctioneer said. Benedict Cumberbatch, who played Turing in the Oscar-nominated The Imitation Game, said: 'Alan Turing was a war hero, who broke the Enigma code helping to bring an end to World War II, the father of modern computing science, and a gay icon who lived in a time of intolerance and tragically committed suicide as a result. 'His impact on our everyday lives is enormous, and the thought of holding a manuscript by him is thrilling.' The notebook was among the papers he left in his will to friend and fellow mathematician Robin Gandy. Turing (left) wrote the notes at the Bletchley Park code-breaking headquarters in 1942 and they paved the way for computer science. The notes are believed to be the only extensive Turing manuscript (right) known to exist, the auctioneer said . Pages of notes show the inner workings of Turing's thoughts as he helped break codes and decipher tests . The notes, which are highly personal, show Turing working through different equations and considering different results . Gandy gave the papers to The Archive Centre at King's College in Cambridge in 1977. But he kept the notebook, using its blank pages for writing down his dreams at the request of his psychiatrist. Bonham describes Gandy's entries as highly personal; the notebook remained in his possession until he died in 1995. At the beginning of his journal, Gandy writes: 'It seems a suitable disguise to write in between these notes of Alan's on notation, but possibly a little sinister; a dead father figure, some of whose thoughts I most completely inherited.' In a statement through Bonhams, Turing scholar Andrew Hodges said the notebook sheds more light on how Turing 'remained committed to free-thinking work in pure mathematics.' Benedict Cumberbatch, who played Turing in the Oscar-nominated The Imitation Game (left), said: 'The thought of holding a manuscript by him is thrilling' In some Turing elaborates on his conclusions, writing that one result 'is too subtle and makes an inconvenient definition' He added: 'Alan Turing was parsimonious with his words and everything from his pen has special value.' Turing committed suicide in 1954. He was gay at a time when homosexuality was illegal in Britain and was convicted of indecency in 1952. He agreed to undergo hormone treatment as an alternative to imprisonment to 'cure' his homosexuality. It will be sold by Bonhams in New York on Monday. It said the seller wished to remain anonymous. Part of the proceeds will be donated to charity.
56-page book contains Turing's thoughts on tricky 'Leibniz notation dx/dy'
It was written at the Bletchley Park code-breaking headquarters in 1942 .
Only extensive Turing manuscript thought to exist, the auctioneer said .
It will be sold by an anonymous seller by Bonhams in New York on Monday .