‘Obviously,’ I nodded. I didn’t know where he was going, but I was on safe ground. Prime numbers can be divided only by one and themselves. I’d at least learned that much in school.

‘But it’s a special prime number,’ he continued. ‘Because it’s the largest right truncatable prime in base ten.’

‘Right.’ I dragged hard on the Marlboro. It had taken him less than thirty seconds to lose me. ‘What does that mean?’

‘It means,’ he said, ‘that if you remove the right-hand integer’ – I cocked my head – ‘uh, number, it’s still prime. So 7393913, 739391, 73939, and so on, they’re all prime numbers, all the way down to 7. It’s really interesting because there are only eighty-three right truncatable primes in base ten, anyway.’

‘Fascinating.’ I blew a cloud of smoke up into the night sky. ‘Tell me,’ I asked, swallowing my pride, ‘are we at the “patterns” stage here, or the “solutions” stage? Because I can tell you how many mils to correct a scoped target at a thousand metres uphill in a stiff breeze. But I’m not exactly sure I could tell you what “base ten” is.’

‘Base ten is the …’

‘What,’ I interrupted, losing my patience, ‘does it mean, Baaz? Why is the fact the number is a “right” whatever important?’ I sucked on the cigarette. ‘Man, you have risked a lot to be here. More than you know. Much more. So please, please tell me – for my sake as well as yours – that there is more to this than a quirk of the Federal Reserve’s printing process.’

‘That’s it!’ He stood up, his voice rising with excitement. ‘You guessed! I knew you would!’

‘Jesus Christ! Would you ever sit down?’ I looked around. Except for the night lights in the bar the joint was dead. He sat again, perched on the edge of his seat. ‘Guessed what?’ He was so excited now that his fingers drummed a constant tattoo in front of him.

‘The Fed. That’s it. Look. Look at the bill.’

‘Sure. I will. Later, though.’

‘No, now,’ he urged. ‘Let’s look at it now. I can show you.’

With misgivings, I stubbed out the cigarette and removed the note from my pocket. Possession of the bill was, it seemed, both a death sentence and stay of execution. If we were being watched, the location of the note would be beyond doubt. But with the high wall of the hotel behind us and the sea in front we weren’t directly overlooked and there was no obvious position for a snooper, or a sniper, to take up. I unfolded the bill and we sat closer. I struck another match and held it over the ragged slip of printed paper.

‘OK, what am I looking at?’

‘Here,’ Baaz pointed. ‘These two letters, before the serial number: LL. The first L is the series of the banknote. See, here at the bottom, 2009A.’

‘OK.’