Disclaimer: Not my boys, alas.

Disequilibrium

Late afternoon sunlight poured through the office windows, as thick and warm as melting butter. It brushed extra gilt across the spines of the classical mathematics references on the bookshelves--Principia Mathematica, The Elements, Disquisitiones Arithmeticae, Flatland. It coated the scattered toys with a layer of translucent gold and struck sparks off the clouds of chalk dust that hung thick in the air, giving the room its own subtle shimmer. It spilled across the desk of Professor Charles Eppes, who remained oblivious to the tranquility of the scene.

Instead, Charlie grunted in annoyance and repositioned the stack of papers in front of him so that the glare of sunlight off mid-terms wasn't quite so harsh. Darla, his grader, had called in sick--spring fever, most likely--and the tests wouldn't grade themselves. He consoled himself with the thought that he had probably seen every single way a student could mess up converting Cartesian to polar and spherical co-ordinates at least--to resort to idiom--a million times. He had Blind Melon cranked up on the MP3 player and a stockpile of bottled water in his bottom desk drawer. He could do this.

Charlie flipped a completed test onto the pile on the floor by his chair and pulled the next one over. He caught a glimpse of pink and sighed. Trevor and his boyfriend Walter both insisted on bringing their pink pencils and coloring in every div symbol. Politicizing mathematics made Charlie uneasy, but he could never bring himself to complain. He much preferred the tiny, exquisite caricatures of the other faculty members that Akim liked to draw impaled on the terminating arrows of his x, y, and z axes. Someday, when he got the nerve, he'd ask one of Akim's other professors what he looked like--

A screaming guitar chord made him flinch, and Charlie reached for his MP3 player without looking up. Yes, it had been sweet of Amita to record some of her favorite music for him, and yes, he'd been stupid to let her.

The MP3 player skittered away from his fingers and slid over the edge of the desk. Both earbuds popped out--well, that was something, at least--but he winced as he heard the sharp click of the MP3 player hitting the floor. He closed his eyes and let his head thunk onto the test. This is certainly a day we're having here. He ducked down and scrabbled under his desk.

His hand came down on the warm, slightly rough skin of another hand.

"Shit!" Charlie shoved away from the desk, sending his chair rolling backwards into a cabinet. Paper airplanes rained down.

"Looking for this?" said a dry, amused voice. Charlie groaned.

His brother Don grinned at him from the other side of the desk. Don's chin was just level with the desktop; he waggled the MP3 player at Charlie, then smoothly levered himself up to perch beside the stack of tests. He swiped an earbud across the sleeve of his dress shirt and held it to his ear. He winced. "Geeze louise. Amita?"

"What are you doing here? How long have you been--why didn't you say something?"

Don set the MP3 player down and surveyed the room. His gaze lost focus and Charlie saw the exhaustion his brother's smile had been hiding. Then Don pulled in a deep breath and turned to Charlie, slipping his smile back on like a Kevlar vest, and raised a hand in a "scout's honor" gesture. "Swear to god, Charlie, I was just going to wait until you finished with one of those papers. But you were so clueless--" his smile broadened to a devilish grin-- "I couldn't resist. I had to find out how long it would take you to notice. Which reminds me." Don glanced at his watch. "Approximately seventeen minutes and fifty seconds. Not bad, Chuck."

"We've been talking--"

"Already taken into account."

Charlie scooted his chair up to the desk and snatched the MP3 player from Don's fingers. It looked none the worse for the impact--more than he could say for his nerves. Larry had once opined that having one person who could still make Charlie feel like an idiot was a good thing. It would help him keep his sense of perspective. But did that one person have to be Don?

Don's hand landed on his shoulder and Charlie started, almost dropping the MP3 player again. He looked up.

"I'm sorry, buddy," said Don, and again Charlie sensed his brother's exhaustion. "To be honest, it was nice to just sit and enjoy the quiet for a bit."

Charlie nodded and gently set the music player down as he searched for something to say. He wanted to hover, to fuss, to invoke decent meals and adequate sleep; he'd wanted to for weeks, now, as one tough case after another battered his brother--shots taken and not taken, lives lost, lines crossed. But he'd watched their mother try to fuss during high school when Don would come home after spending a few hours post-baseball practice in the batting cage, wolf down left-overs, and then rush upstairs to do homework. It had never worked for her. He cleared his throat. "Do you have something for me?"

For a moment Don's hand felt heavy on his shoulder and he regretted asking. Then Don gave Charlie's shoulder a last pat and slid off the desk. "Yeah. As a matter of fact, I do. Nasty new drug on the streets." He retrieved a manila folder from the chair directly in front of Charlie's desk--presumably the one he'd been sitting in while Charlie continued to grade tests, oblivious to his presence.

"You were that close?" Charlie glared at his brother. "For seventeen minutes and fifty seconds?"

He was rewarded with another faint smile as Don hooked the chair with a foot, drew it even closer, and sank into it. "Give or take a few."

Don tossed the folder on the desk, obscuring Trevor and Walter's political statement, and Charlie picked it up. The folder held little more than a few tox screens, interview write-ups, and a map of the greater Los Angeles area with a scattering of seven red dots sprinkled across it--most in Northridge, two trailing down to Reseda, a possible outlier in Granada Hills. With an effort he stopped assigning even tentative qualitative significance to the points; without the data, he didn't want to chance corrupting his own thought processes. Under the map he could feel the outlines of smaller, stiffer sheets. Victim photos. Charlie closed the folder and looked up at Don.

"I know, buddy," Don said quietly. "Not much to go on. This got dumped in my lap a few hours ago."

"I thought you were still working on all the paperwork from that arson case."

Don shrugged. "No rest for the wicked," he said. "I want to get a handle on this as quickly as possible. It's--nasty." If Charlie didn't know his brother, wasn't watching for the signs, he might not have noticed the hitch in Don's voice or the effort it cost him to meet Charlie's gaze. Charlie frowned and flipped to one of the tox screens as Don continued. "It's being sold as Ecstasy, but we haven't seen this particular pill before. It's cut with--"

"Brodifacoum? What's that, Don? I've never heard of it."

Now Don did look away. "Rat poison," he said tightly.

Charlie gasped and turned back to the tox screen. Once he knew what to look for, the symptoms screamed at him: massive internal bleeding, hypovolemic shock, multiple organ failure. He flipped to the case overview as Don continued. "According to survivor statements, we've got a new guy in town, selling a new pill--blue, with a bird imprint. Surprise, surprise, the pills actually contain a respectable amount of Ecstasy--about 20 percent by volume."

"Guy? Single?"

"Yeah." Don ran a hand through his short, dark hair and slumped down in the chair, long legs stretched out in front of him. He stared vacantly past Charlie. Rainbows split from sunlight by the prism in Charlie's window danced across his face and Don closed his eyes and held very still, as though he could feel their touch on his too-pale skin. Charlie watched, entranced. "Witness descriptions all match. Mid-to-late twenties, brown eyes, short brown hair, about five-ten. Showed up with his goodies two days ago. So far we've found five locations that he hit, all in Northridge and environs."

"Two days? That was a Monday. Isn't Ecstasy a party drug? Seems like a funny thing to buy on a Monday."

"He was selling cheap. Called it an early bird special." Don grimaced. "That's what the kids are calling these pills--early birds. A few of the kids decided to try them out Monday night." He opened his eyes and Charlie swallowed at what he saw there. "Early birds."

Charlie refused to follow that lead. "He could just be the distributor."

Don shook his head. "I've got a feeling about this one. You want to make money, you cut with something inert--at the very least, non-toxic. This guy wants to kill kids."

Charlie rubbed his forehead, then weighed the folder in his hand. It was so thin--too thin. He couldn't tell his brother there wasn't enough data. He had to tell his brother there wasn't enough data. "Is there anything else about this case that you can tell me? Something that's struck you, that you can distill out of the papers in this folder? Anything about the victims? Anything about the locations? Anything?"

Don straightened and eyed Charlie, frowning slightly.

Charlie shrugged. "I've got my intuitions about numbers, and you've got yours, about--"

"Psychos?"

Charlie ignored the interruption. "Crime and criminals. Right now, with so little to go on, I really need you to put your intuition to work."

"Well--" Don hunched forward, elbows propped on knees, and stared at his clasped hands. Rainbows played in his hair. He looked up. "There's something very weird about the different pills we confiscated. Not all of them contained the same amount of Brodifacoum."

"Maybe he just started running out."

"Then he got a piss-poor amount to begin with."

"Okay, okay." Charlie flipped through the tox screens again, mentally correlated percentages to map locations. "I can try treating it as a basic mixing problem just to get started," he said. "I can come up with an ODE, play with parameters, generate some families of solution curves, but--"

"ODE?" Don's voice was still quiet, but a note of strain had entered it.

Charlie glanced at him. "Ordinary differential equation," he said. "A measurement of the rate of change of one quantity with respect to another, like velocity is the rate of change of position with respect to time. In this case, I'd try to see how at the percentage of contaminant in the pills changes in relation to--um, maybe a distance from some origin? I don't know yet. To be honest, the math isn't very interesting, but by that same token it won't take me very long. As soon as we get more data..."

Don had gone very still, and Charlie knew his brother well enough to realize he'd said something wrong. There was a grim set to Don's mouth and a speculative look in his narrowed eyes. Charlie suddenly realized what it must feel like to sit on the other side of an interrogation table from his brother. "I--Don?"

"Maybe if you think of your ODEs as ODs and your data points as dead kids the math might get a little more interesting."

Charlie sucked in his breath as a cold tide swept through him. After thirty years, he could convince himself that he was used to Don yelling, but his brother's quiet rages never failed to unnerve him. "That's not fair, Don. I'm thinking like a mathematician. That's what you pay me to do."

Don looked away and the moment passed. He scrubbed his face with one hand and blew out a long breath. "You're right. I'm sorry. I just hope that was Ivory Tower Charlie talking, not Jaded Charlie."

Charlie blinked as the cold wave receded, leaving him feeling shakier than he wanted to admit. He looked down at the folder, thought about how he'd refused to flip to the victim photos. "Ivory Tower Charlie," he whispered. "I think."

"Good." Don peered at his younger brother and something in Charlie's face made him raise a hand, let it drop. "Maybe I'm selfish, but as much as it bugs you to look at it, I wish you weren't getting used to this stuff. Because after a while you realize that you're actually not. Used to it, I mean." Don shook his head once, sharply, and grabbed the folder. He stood. "Maybe I should just give you a break--"

"No! Leave it. It's--it's okay."

Don stopped, wavered. "I'm going to hell for this," he said, and dropped the folder back onto Charlie's desk.

"Don?"

"Yeah, Charlie?" Decision made, Don checked his watch and turned toward the door.

"Are you still seeing that--that--"

"Bradford? The shrink?" At Charlie's nod, Don hesitated for a moment, glanced at the chair, then the door, then compromised by perching on the corner of the desk again. He eyed Charlie warily. "Yeah. Why?"

Charlie felt like rolling his eyes, but Don seemed to be in an oddly balanced state of depressed-but-not-too-depressed that left him willing to talk, and Charlie didn't want to add a frisson of irritation that might chase him away. "Is it--helping?"

Don looked at Charlie, snorted, shook his head. "I don't know. Maybe. I mean, you spend a lot of years purposefully not thinking about stuff, and then somebody wants you to think about stuff." His gaze came to rest on the top of Charlie's desk, and Charlie had to wonder what his brother was seeing reflected in the scarred wood. "It--it makes you think."

"About stuff?" Charlie asked softly.

Don glanced up and the corners of his mouth twitched. "Yeah. About stuff."

"Look. What do you say I gather up these tests and you take me home? You can hang out with Dad, maybe watch a game, and I can grade for a while and then work on your case."

"Thanks, Charlie, but I've really got to be getting back. I left Colby with the paperwork."

"Which Colby can do--"

"--except he spells about as badly as you do. Gotta maintain a certain level of professionalism in the office, you know. Besides, you don't want your gloomy brother hanging around all the time."

"Don't mind at all. You don't talk much when you're gloomy. I can get my work done."

"Ha. What about Larry? You can't have heard all his stories yet."

Charlie allowed himself a small smile. "True, but Megan gets first dibs."

Don nodded, a smile teasing the corners of his mouth as well. "Amita?"

"She's in Hawaii, at an astrophysics conference. She went for Larry. They're getting a tour of all the scopes at the top of Mauna Kea and he was afraid he'd get altitude sickness."

"Larry's afraid he'll get altitude sickness?" Don grinned. "He just came down from how high?"

"Well, the ISS orbits between roughly 320 kilometers and 350 kilometers above mean sea level, but it's actually a matter of atmospheric pressure--"

And Don finally laughed. Something in Charlie eased at that and he smiled up at his brother. Don pushed himself to his feet. "Tell you what--save me some dinner and I'll try to stop by later."

Charlie nodded. It was all he'd get out of Don, he knew. As his brother slipped out of his office, Charlie sighed. "No rest for the wicked," he murmured. Or for Don, either.