Dirk Gently

'It's just that it's stuck halfway up a long flight of stairs which leads up into my flat. As far as I can make it out, the delivery men got it part way up the stairs, got it stuck, turned it around any way they could, couldn't get it any further, and then found, curiously enough, that they couldn't get it back down again. Now, that should be impossible.'

    'Odd,' agreed Reg. `I've certainly never come across any irreversible mathematics involving sofas. Could be a new field. Have you spoken to any spatial geometricians?'
`I did better than that. I called in a neighbour's kid who used to be able to solve Rubik's cube in seventeen seconds. He sat on a step and stared at it for over an hour before pronouncing it irrevocably stuck. Admittedly he's a few years older now and has found out about girls, but it's got me puzzled.'
    `Carry on talking, my dear fellow, I'm most interested, but let me know if there's anything I can get you. Port perhaps? Or brandy? The port I think is the better bet, laid down by the college in 1934, one of the finest vintages I think you'll find, and on the other hand I don't actually have any brandy. Or coffee? Some more wine perhaps? There's an excellent Margaux I've been looking for an excuse to open, though it should of course be allowed to stand open for an hour or two, which is not to say that I couldn't. . . no,' he said hurriedly, `probably best not to go for the Margaux tonight.'
    `Tea is what I would really like,' said Richard, `if you have some.'
    Reg raised his eyebrows. `Are you sure?'
    `I have to drive home.'
    `Indeed. Then I shall be a moment or two in the kitchen. Please carry on, I shall still be able to hear you. Continue to tell me of your sofa, and do feel free in the meantime to sit on mine. Has it been stuck there for long?'
    `Oh, only about three weeks,' said Richard, sitting down. `I could just saw it up and throw it away, but I can't believe that there isn't a logical answer. And it also made me think - it would be really useful to know before you buy a piece of furniture whether it's actually going to fit up the stairs or around the corner. So I've modelled the problem in three dimensions on my computer - and so far it just says no way.'
    `It says what?' called Reg, over the noise of filling the kettle.
    `That it can't be done. I told it to compute the moves necessary to get the sofa out, and it said there aren't any. I said "What?" and it said there aren't any. I then asked it, and this is the really mysterious thing, to compute the moves necessary to get the sofa into its present position in the first place, and it said that it couldn't have got there. Not without fundamental restructuring of the walls. So, either there's something wrong with the fundamental structure of the matter in my walls or,' he added with a sigh, `there's something wrong with the program. Which would you guess?'"

...

"At the other end of the room were a couple of long tables smothered in, at the last count, six Macintosh computers. In the middle was the Mac II on which a red wire-frame model of his sofa was lazily revolving within a blue wire-frame model of his narrow staircase, complete with banister rail, radiator and fuse-box details, and of course the awkward turn halfway up. The sofa would start out spinning in one direction, hit an obstruction, twist itself in another plane, hit another obstruction, revolve round a third axis until it was stopped again, then cycle through the moves again in a different order. You didn't have to watch the sequence for very long before you saw it repeat itself. The sofa was clearly stuck."