David Auburn's Proof promises much more than it ever quite delivers. A mathematical-psychological puzzler, a second viewing shows it to be a cracker on the former but less convincing in the latter: a sort of paler shade of Michael Frayn - one has only to think of Frayn's Copenhagen to remember what an explosive dramatic mixture mathematics and psychology can create.

Auburn's Pulitzer Prize-winner - the original opening five years ago saw Gwyneth Paltrow starring in her first West End role at the Donmar - is, however, of more fragile, even evanescent material. A mathematical genius who loses his mental abilities, a daughter who cares for him while teetering herself on the brink of genius or similar instability, some sibling rivalry and a young academic who fails to show sufficient trust - Auburn's ingredients are delicate flowers that at any moment threaten, theatrically, to implode before our eyes.

Birmingham Stage Company's transferred production (it was a sell-out in Birmingham) from veteran director John Harrison shows admirable restraint and sensitivity to the clashing chords of this familial tug-of-war and observation of the mathematical mind. But despite Auburn's structural daring, colliding past and present with recently deceased Papa, Robert, brought back to converse with loving daughter, Catherine, this production suffers from an almost fatal lack of variety.

Once we have been introduced to designer Norman Coates' clapperboard verandah set (it looks more East Coast than the supposedly Chicago home of a university academic) and Jo Dawson's lighting, that is it. Little else changes visually throughout the play's two hours traffic, which would be fine if we were emotionally engrossed in the battle for Catherine’s soul. But the younger actors here seem so intent on imitating the style of laid-back Americana, clearer definition flies out the window.

Sally Oliver making her London professional debut as the daughter who gave up her own chances to care for her wayward genius father, (a fine Terence Booth, full of whirling words and deceptively reasonable behaviour) exudes an opaque blankness appropriate for a character whose tenuous hold on reality is always in doubt.

As the Ph.D student, Hal, whose own motives are also subject to a certain ambiguity, Neal Foster offers up an appealing and convincing study of charm with just a hint of duplicity, and Aislinn Sands as the New York sibling, Claire, gives an excellent impersonation of fear held at bay by hyper-control. At the end, though, I still felt here was a play in search of a defining motor, dying for lack of theatrical oxygen.

- Carole Woddis