Lecture Notes For Linear Algebra Gilbert Strang May 2026

“This is the twelfth –” the headmaster glances up from his notes – “no, let me correct that – the thirteenth time you’ve been in trouble this term, Agatha.”

We’re sitting in his office, the air sticky, and that’s not just because of the heatwave outside.

I look down at the floor. It’s true, and I don’t know what to say.

Doctor Hargrave (Ronald Hargrave, BPhil MEd OBE) likes to fill silences. He’s very good at that, and it’s best to wait until he’s done. He isn’t a doctor, as you and I think of them, but he likes to be called one. He has five liver spots in the shape of the constellation Cassiopeia on his forehead, and a steely glare which I would say is a 4B on the eye colour chart I have hanging in my bedroom.

He reads from his list:

“One – you were found hiding in the ceiling space above the chemistry labs, because you believed Mr Stamp was stealing sulphuric acid to sell on eBay.”

This really happened – he was – but without evidence I had to drop my investigation. Plus, Dad grounded me.

“Two – you tried to miss lessons by convincing the groundskeeper that you were an apprentice tree surgeon who needed to scale a tree near the boundary wall… and just so you could get out of school…”

I zone out. I’ve always found this easy – like switching channels on TV. If I want to watch something more interesting, I just imagine it. I call it my ‘Change Channel’ mechanism.

The headmaster’s desk is very shiny and if I look down I can see my own reflection in the caramel-coloured wood. I’m wearing my red beret, my bob-cut hair framing my face, and my eyebrows are knitted together as though concentrating on his lecture. And, just like that, my reflection shimmers, shifts, and becomes someone else. A small man in a hat and a bow tie looks back up at me. Smoothing out his moustache, he steps out of the desk, hops neatly to the floor, and stands behind the headmaster.

“How long do you think Monsieur Hargrave will go on this time?” he asks in a soft Belgian accent.

I zone back in to hear what my headmaster is saying now…

“Four – you installed a listening device in the wall of the staff room…” – and then I glance back to where Hercule Poirot, famous detective, is glancing at the clock.

“Your headmaster has already been talking for twenty-two minutes.” Poirot raises an eyebrow, as though daring me to do something about it. “He might break his record of twenty-seven, no?”

Actually, I reckon the headmaster is almost done – his stomach just rumbled, and it’s long after lunchtime. My eyes flicker around the room, details lighting up my mind like a pinball machine.

NEW HAIRCUT

STOMACH RUMBLE

EXPENSIVE SHIRT – SILK

CHOCOLATES – A GIFT

DARK GLASSES

“Twenty-four,” I say out loud.

“What?” The headmaster looks up from his notes.

“Nothing.” I clear my throat.

Poirot nods in recognition – I have made my bet.

“Are you listening to me, Agatha?”

“Absolutely, sir. You were saying that impersonating a health inspector is a criminal offence.”

“Yes, I was. Do you not take that seriously, Agatha?”

I nod, seriously. “I do, Headmaster. I was just starting to worry.”

“Worry? Worry about what?” The headmaster’s eyebrows furrow.

“That you’d be late for lunch with your wife.”

A look of confusion creases his face at the change of tack. “My… wife?”

“Yes. You’re wearing a very nice shirt, sir. And aftershave. And I couldn’t help notice the box of chocolates on your table, clearly a gift for a lady…” I smile, pleased with my investigatory skills.

“Aha, yes,” he splutters, “my wife.” He looks at the clock on his wall. The words hover in the air like fireflies. “As you were saying, I’m going to be late for lunch… with my wife.”

“Well, sir, I wouldn’t want to make you late,” I say.

Dr Hargrave stands up, brushing invisible crumbs from his suit. “Yes. I’d better get going.” He glances around, as though looking for the exit. “As for you, Agatha, I would advise you to think about… um… everything I’ve said.”

“I will, sir.”

Dr Hargrave seems to be sweating as he shows me to the door where Poirot stands, smiling with approval. Poirot looks at his pocket watch.

“Twenty-four minutes – you were right, mon amie.”

BREAKING NEWS: MURDER AT BRITISH MUSEUM

I stare at the red letters for a moment, feeling a familiar excitement. It’s been five weeks since I solved the case of the red slime that had polluted London’s water supply – and I’m itching to get going again. Things have been too quiet, with no cases to solve, so I haven’t been enjoying my summer holidays as much as usual.

I take his phone and click on the link. There’s not much information to go on:
A member of staff has been stabbed to death shortly after closing time this evening at the British Museum. Police have yet to release the name of the staff member, who is believed to have disturbed an intruder. A museum exhibit is said to be missing from a display case.
I feel a surge of happiness. ‘Finally, an actual case!’ I catch his eye as I hand back his phone. ‘Come on – we need to investigate!’
He scrutinises me. ‘Agatha, tell me you’re not actually pleased that someone’s been murdered…?’
My cheeks turn red; hopefully he hasn’t noticed. ‘Of course not.’ I study my nails: currently black with silver stars. I’m especially pleased with the stars, which came out just right.

‘Anyway,’ he continues, ‘it’s a murder investigation – you can’t just wander in there!’
This is the type of challenge I live for. ‘Of course I can. No Case Too Odd, remember?’ I say, reciting the motto for our detective agency, Oddlow Investigations. I’d got sick of people making fun of my surname, Oddlow (Oddly, Oddball, Oddbod, Odd Socks...) – so I’d decided to put a positive spin on it. ‘But it doesn’t even seem especially odd...’ he says, doubtfully. Not wanting to waste time, I grab him by the arm and start to stride through the Leicester Square crowds, in the direction of the Tube station. Liam stumbles after me, reading the story on his phone.
‘It says they’ve put the museum on lockdown. Nobody can get in there, not even you.’ ‘Ah, but who said I was going to use the front door?’ I look back at him, raising an eyebrow.