Mariah Gale & Matthew Marsh in Proof (photo: Nobby Clark)
This Pulitzer-winning play from David Auburn, first seen in London in 2002 in a production at the Donmar starring Gwyneth Paltrow (who went on to star in the 2005 film version), is not quite as clever as it could be.
Faithfully revived at the Chocolate Factory in an unfussy production by Polly Findlay, it now seems rather behind the times when held up next to the likes of Complicite's A Disappearing Number or, more recently, the National's Curious Incident of the Dog in the Night-time.
But to compare it to these productions on the grounds it centres around a maths problem is perhaps unfair, because of course like all plays about maths problems it's not really about maths at all. It's about the relationship between a father - who happens to be a genius gone mad - and his two daughters.
One of those daughters, 25-year-old Catherine (Mariah Gale), has nursed her eminent father Robert (Matthew Marsh) through his final years in Chicago, absorbing a significant chunk of his maths ability in the process. The other, Claire (Emma Cunniffe), has decamped to New York with her husband and returned to try and persuade her sister to let go of the house.
The spanner in the works comes in the shape of grad student Hal (Jamie Parker), who harbours equal passions for Catherine and her father's problem-solving abilities. So his discovery in a desk drawer of a brilliant 'proof' involving prime numbers causes something of a stir, especially when Catherine claims to have authored it.
The action plays out on the back porch of the rather shabby family home, superbly rendered by designer Helen Goddard. Opening with an exchange between Catherine and her father's ghost, Auburn introduces the notion that much of what passes between these two characters is laden with mystery - not least whether his psychological issues are also affecting her.
He skilfully plays in the oft-stereotyped territory between genius and madness, though the denouement comes as a disappointment considering the depth of the conflicts established to that point. Would the ambitious Hal not be more perturbed by the idea that his beloved is a greater mathematician than he?
But there is nothing disappointing in the performances. The painfully fragile Catherine fits Gale like a glove, while Parker plays preppy to a T. And the ever-reliable Marsh, thankfully given much more to do in the second act than the first, poignantly portrays a once great man battling the fading light, struggling to make the numbers work and, more importantly, remember his daughter's birthday.
A solid evening all told, but not exactly a head-scratcher.