The Indian Clerk, By David Leavitt

Mathematicians and novelists inhabit their imaginations to different ends. Mathematicians shape the abstract world of numbers into proofs; what is proved is true, and what is true might be real. Novelists plunder the material world for facts to create an illusion which might be truer than appearances.

Proof, truth and honesty are at issue in David Leavitt's re-imagining of the life of the mathematician GH Hardy. The motor of the novel is Leavitt's scepticism over the claim that Hardy was a "non-practicing homosexual". Leavitt refuses to believe there can be any such disposition and has great fun making hay of the two-faced attitudes to sexuality enjoyed by England's privileged between the Wilde trial and the First World War.

The University of Cambridge of the period supplies a list of stellar personalities to criss-cross Leavitt's loose and episodic narrative: John Maynard Keynes, DH Lawrence, Wittgenstein and Bertrand Russell. The novel pivots around Ramanujan, the "Indian clerk" of its title, a self-taught genius whom Hardy lifted from the obscurity of the Madras post office to the courts of Trinity College.

In factual accounts, such as Robert Kanigel's The Man Who Knew Infinity, Hardy is cast as the cold fish to Ramanujan's fish out of water. In inventing a furtive love life for Hardy, Leavitt makes both men a little rounder. Parts of the novel move in a crisp historic present tense, and parts are presented as an inner monologue from Hardy – the thoughts he feels he cannot express as he presents a lecture on Ramanujan at Harvard in 1936. The Ramanujan Hardy recalls is bumptious and petulant as well as a genius.

Leavitt is perhaps too harsh on his reimagined Hardy, impatient with him for not coming to terms with the subterreanean sex life that is entirely Leavitt's own invention. The novel is as well researched as any biography (with as many petty errors as one would expect to find in the work of a professional historian) but Leavitt allows his imagination to colour his judgement. Because the imaginary Hardy distances himself from his more flamboyant contemporaries such as Lytton Strachey, Leavitt infers that Hardy's pacificism during the First World War was timorous and half-hearted. This has more to do with the radical politics of the 1980s, when gesture was all and the outrageous berated the conventional for their supposed inauthenticity, than the early 20th century.

The almost wholly imagined Alice Neville also represents a contemporary ideal of what a Cambridge don's wife of the time should have thought, felt and done. Gertrude, Hardy's sister, is splendidly imagined, and the relationship between a brother and sister who maintain a warm affection through a limited emotional vocabulary is well mapped out.

Leavitt has a deep affection for all his characters and if his imagination runs to wish- fulfilment on their behalf, when it finds a soldier lover for Hardy, for example, the history may suffer but the novel does not. Leavitt's enjoyment is infectious and he makes us understand that although Hardy and Ramanujan found it easier to love numbers more than people, they loved numbers very much indeed.