Oedipus, National Theatre: Olivier, London A disappearing number, Barbican, London

A harrowing and fateful voyage round his father

It's more than a decade since the National staged Sophocles's great primal tragedy. The difference of approach between the two productions is stark. In 1996, Peter Hall went for broke, renewing the ancient Greek practices of mask and ritualised movement and using a translation of rhyming couplets. Jonathan Kent's new Olivier production shows what can be achieved with a style that mediates between formality and modern informality.

The chorus, for example, are citizens in contemporary suits who can turn into a male voice choir for passages of dissonant lamentation and protest. The setting of verdigrised brass is both monumental and undercutting – huge double doors mounted on a curved disc of a stage that gives the characters no flat ground. Frank McGuinness's translation is a mix of impassioned simplicity and sly, modern notes of self-conscious irreverence – as when, knowing the relief that it will bring, the Stranger from Corinth tells Oedipus that his father is "Dead and gone/ Done and dusted".

As a programme puts it, Oedipus is perhaps the one person not to suffer from the complex Freud named after him. Since hearing the fateful words of the oracle, he has done all he can to distance himself from possible parricide and incest. So is he the innocent plaything of the god? This production takes the line that the hero's fault is an overweening belief that, having solved the riddle of the Sphinx, he knows the score where knowledge is concerned. Excellent Ralph Fiennes exudes the defensive hubris of a man bent on satisfying his curiosity, even when the truth is imploding him from within. It's impossible to withhold pity, though, from the stricken figure who afterwards beseeches his terrified children to lead a life better than their father's.

The production has strength in depth. Alan Howard brings a mocking Beckettian Irish brogue to the blind seer, Tiresias. As they struggle to fend off the full horror of their position, Fiennes and Clare Higgins's superb Jocasta fall into gestures of mutual consolation that look hideously like eroticised versions of the mother-son relationship. An impressive, harrowing evening.

Conceived and directed by Britain's greatest theatre-maker, Simon McBurney, Complicité's A Disappearing Number won all last year's awards for best play. It's back at the Barbican, and even better. Finding deep metaphors in mathematics, the show interweaves stories around the real-life collaboration between the Cambridge don GH Hardy and the young, self-taught Brahmin genius, Srinivasa Ramanujan, who defied his caste and accepted Hardy's invitation to come to work with him in England at the time of the Great War.

In the production's first incarnation, this primary story – which ends with the early death of Ramanujan back in India – seemed to take second place to a complementary narrative about Al, an American futures dealer, and his wife Ruth, a mathematician besotted by Ramanujan and desperately aware of the short shelf-life of maths boffins and the inexorable ticking of the biological clock. Without at all demoting that strand, this version gives a much more involving texture and urgency to the Hardy/Ramanujan collaboration. We see them, for example, working at fever pitch trying to find a predicative formula for the way numbers "partition" or subdivide within themselves – with all the symbolic applications of that concept to other areas of life.

A miracle of multimedia poetry, the piece fervently dramatises Hardy's fundamental perception that "a mathematician, like a poet or a painter, is a maker of patterns". Complicité's great forte is for taking ideas that could seem abstract and remote and uncovering the visceral connection between them and the psychological and emotional. We witness the infinitely convergent series of numbers become an emotional metaphor for a couple anxiously wanting a child, or ponder whether nought divided by itself equals one or infinity. Essential viewing.

'Oedipus', in rep to 4 January (020-7452 3000); 'A Disappearing Number', to 1 November (020-7638 8891)