## Vertices

### Work Text:

"Don!" Charlie called out from behind him. "Don, wait up a second."

Weaving his way through the cubicles towards his, Don didn't even slow down. He did raise his hand to rub at his eyes, and then at one temple, briefly. Charlie had the best intentions, he really did, but at the end of a long day his energy wasn't invigorating or endearing or even amusing. It was just exhausting. Sometimes even irritating to the point of grating on Don's nerves. Not answering Charlie's calls might not be polite, but it was a lot better than snapping at him.

But once Don got to his cube, there was nowhere left to go, and Charlie just crowded up close to his desk. "It worked right?" he asked, eyes sparkling, all but bouncing on the tips of his toes. "You got them?"

Don dropped down into the chair behind his desk. "Yes," he said tiredly. "It worked. God only knows why, but they were right where you said they'd be. Took down all five without even a shot fired. Megan and Colby are downstairs processing them as we speak."

"You know, I could explain why, if you'd just give me a couple of minutes," Charlie said. His tone was almost diffident, but even as he spoke he was reaching for the notepad on Don's desk and a Sharpie that was sticking up out of his pen holder.

"Charlie--" Don began, but his brother was already off and running.

"It's all areas of optimization," he said, intent on the paper, diagrams and equations spilling from the pen as if they'd been lurking in the ink. "A Hamiltonian path, specifically. Basically, a Hamiltonian path is an undirected graph which can only visit each vertex once. Of course, first we have to determine the number of vertices in the graph. If you consider each location the suspects were required to visit to be a vertex--"

"Stop, Charlie, just stop," Don said, putting his hand down on the piece of paper and getting a black streak across the back of it for his trouble. "I don't care how it works as long as itdoes."

"But--" Charlie protested.

Don cut him off. "I'm never going to understand this stuff, Charlie. It just makes my head hurt when I try."

"I don't see why you can't at least understand when I explain. With your memory, it should just be a matter of recall." The hell of it was, Charlie looked honestly confused.

"Perfect memory and perfect comprehension aren't the same thing." Don snorted. "If they were, I'd have done better in school."

"You didn't do badly," Charlie offered, turning the marker over in his hands.

"Good enough to get into Quantico." Don smiled briefly. "Better than you in English."

Charlie made a face at him. "I still think you could get it if you just applied yourself a little more."

"Applied myself-- Charlie, you're not Dad. I'm a grown up. I get to apply myself to the things Iconsider important." The moment the words were out of his mouth, Don had to conceal a wince. Talk about hitting one of Charlie's hot buttons.

Sure enough, Charlie's hackles went up and his jaw hardened. "Math is important, Don. The past two years should have taught you how universal its applications are."

"I'm sorry, Charlie," Don interrupted the flow of words. He rubbed a hand over his face. "I didn't mean that the way it came out. It's been a long day, I'm tired, and I still have to write up a report on this case. Go home. I'll talk to you tomorrow."

Charlie nodded slowly and started to leave, but he paused just a few steps away. "You're not coming over for dinner tonight?"

He'd meant to. Somehow it'd been nearly a week and he did like to check in on Charlie and their dad every few days. Not that he let them know that's what he was doing. But he just didn't have the energy for it right now. "Not tonight," he said, ignoring the faint twinge of guilt. "Plan for me tomorrow, okay?"

"Sure," Charlie said, nodding, but he made it sound like he wasn't going to take it for granted. Don didn't call him on it, just watched him go and made a mental note to make damn sure he showed up for dinner the next day. Sighing, he leaned so far back in his chair that it wobbled threateningly and closed his eyes for a long moment, blocking out the rest of the office. Hopefully the impending headache would recede if he could just relax for a minute. Thankfully the office was starting to quiet down now.

Eventually he cracked an eye open and looked at the clock on his desk. 6:29pm. He was supposed to call Mac today, but the way this case had suddenly shifted into high gear, there hadn't been a moment yet. He briefly considered skipping the call entirely; having finished checking the day timer over the course of three phone calls, they'd only been planning to take a stab at Stefanos's client files. Not to mention that it was 9:30pm in New York and chances were good that Mac had already given up on him and gone home. But...well, Don had said he'd call. Even if Mac wasn't in, Don could at least leave a message and let him know that he'd hadn't just forgotten.

Still leaning back in his chair, Don dialed and closed his eyes again as he held the phone up to his ear. I ought to get a head set for the office, he thought randomly. In his ear, the phone rang. But only once. There was a click almost immediately and then: "Taylor, NYPD CSU."

Don blinked. "It's Don," he said. "You're still there."

"Obviously," Mac said dryly.

"Please tell me you weren't waiting for me to call."

Mac chuckled. "I wasn't waiting for you to call," he said. "Half of the next shift is out sick, so most of us are working overtime."

"Good," Don sighed. "Well, not good, but-- Hell, you know what I mean."

"If this is a bad time, I can call back," Mac offered.

Don had to laugh, if tiredly, at that. "I called you, remember?"

"That doesn't mean it's not a bad time. Given the time in L.A., I'm guessing you didn't have a routine day."

"If there is any such thing as a routine day, today was definitely not it," Don agreed. He rolled his head, listening to his neck crack, and stared up at the ceiling.

"Rough case?" Mac prompted.

"Not the way you mean it," Don said. "It just-- You know how you can work a case for a week, feeling like you're running in place, and then suddenly everything snaps into place and no matter how fast you work, you can't seem to keep up? That was today."

"Everything taken care of now?" There was a note of concern in Mac's voice that made Don blink in surprise.

"Yeah," he said automatically. "We got the guys. Thanks to my brother."

"I didn't think your brother was FBI." Don could literally hear Mac's frown. He could imagine why; Mac had an excellent memory. Forgetting something like that could be unnerving.

Fortunately, Don could reassure him that it wasn't his memory at fault. "He's not. Charlie's a mathematician at CalSci."

"And he was involved with this case of yours because...?"

"Charlie's specialty is applied mathematics. He consults with us pretty regularly, using mathematical theories to find patterns in the crimes we're investigating. He's been able to predict our suspects' next move on more than one occasion. Sometimes he's even helped to identify the suspect themselves." The explanation rolled off Don's tongue automatically. He'd had far too much practice with it.

"I take it that's not the first time someone's asked that question," Mac said dryly.

"Not even close." Don grimaced in memory. Cops who wanted to know what Charlie was doing on a scene, lawyers who questioned his role in the investigations, suspicious pencil pushers who wanted to know why Don's brother was entitled to a consultant's fee, fellow agents who ribbed him about his little brother still tagging along after all these years... "But he's been directly responsible for the breaks in so many of our cases that it's worth it."

"Even if you sometimes wish it wasn't," Mac said, just a touch of wryness in his tone.

Don let out a huff of laughter that was really more of a breath. "Yeah, sometimes," he confessed. "I can't help wanting to solve my cases myself. But I can't not use a resource like Charlie."

"You're still the one solving the cases, even if you do ask for his help," Mac argued. "When you get a DNA match or a bit of trace that places your suspect at the scene of the crime, you don't consider the tech to have cracked the case, do you?"

"Is there a safe way to answer that question?" Don asked, smiling.

Mac laughed. It was a warm, brief sound. "Yes," he said, pausing just long enough for Don to absorb the fact that he hadn't said which way was the safe way. "My point is, you're responsible for putting the pieces together. Your brother can give you information, but he can't tell you what it means."

"He tries," Don broke in. "Hell, when he does try, he's usually right. I don't know when it happened, but at some point I lost the ability to do my job without him." Don stopped and let the mouthpiece drop away from his lips, though he kept the phone pressed to his ear. He had to be tired to be piling this shit on someone who was already doing him a favor.

But if Mac minded, it didn't show in his mild tone of voice. "You got to be SAC in Albuquerque without him."

"Sometimes I wonder if I got to be SAC in Albuquerque because I was without him," Don said. He tried to make it into a joke, but somehow it came out sounding too serious anyway. "Charlie was born brilliant. He overshadows everyone around him. Good doesn't look so hot when you line it up against spectacular."

There was a long pause. "People who are born brilliant are given admiration," Mac said at last, "but people who have to work hard to be good at what they do earn respect. I don't know about you, but I'd rather have the latter."

Respect or admiration? Don thought of the way people looked at Charlie when they placed him as the Charles Eppes. He compared that to the way his team looked at him. No contest."Okay, point taken," he said aloud. "But you can't tell me that it isn't a lot harder to be content with being good when you have to look spectacular in the eye every day."

"So why not leave? Make your own place somewhere away from spectacular."

"How do you think I ended up in Albuquerque?"

There was another pause, and when Mac spoke again his tone was careful. "The reason you came back originally doesn't apply anymore, Don. You could take another transfer."

Don scrubbed a hand through his hair, grimacing as his fingers broke up little chunks of gelled hair. How the hell was he supposed to explain? There were people here that were his. His herd. Which meant his responsibility, but it was more than that in a way that was all but impossible to explain even to a human who was in the know. But he tried. "Charlie and my dad...they're my family. I thought I'd started over in Albuquerque. I thought I was on my own. But they didn't stop being mine just because I wasn't living in the same city anymore." Don paused, gathering his words. "When I came back, my mom was dying, my brother and I weren't talking, and I felt like something had fallen into place in my life anyway. I need to watch over them." He paused, smiling ruefully. "Even if they aren't aware of it. And probably wouldn't be too thrilled with the idea if I mentioned it."

"Independence is a quality most people over-value," Mac commented.

Don stilled for a moment. "Yeah," he said slowly. "They do. And they don't even realize it. They work just as hard at setting themselves up to be self-sufficient as they do trying to find a mate to share their life with."

"When the most rewarding relationships are those in which both members draw strength from each other." Mac's voice was quiet. Maybe even a little sad. Curiosity reared its head. Don held himself back from asking, but the pause must have been eloquent because Mac went on. "I was thinking of my wife," he said. "She was one of the strongest people I knew."

Was. "I'm sorry," Don said awkwardly. "I didn't mean to--"

"It's all right," Mac interrupted. "It's been five years. I just still miss her sometimes."

"You have any other family?"

Mac audibly inhaled for a moment before he answered. "Not that I'm in touch with."

Don winced. Real smooth, Eppes. Try to hit a few more sore spots. "And this is point where I give up on conversation and return to work," he said ruefully.

Mac chuckled. "There wasn't some terrible falling out, Don. Our lives just moved in different directions. They had each other and I had the Corps and I think all of us were satisfied with how life worked out."

"You were a Marine?" Don asked, surprised.

"Yeah. A sniper. Well, for a couple of tours," Mac clarified. "Took a little while to figure out that's what I was suited for."

Don had to laugh. "It's a small world."

"Oh?"

"I ran into another Marine sniper a few months back," Don explained. "He, ah, also did me something of a favor. Seems like a funny coincidence."

"Name?" Mac prompted, sounding curious. "Maybe I know him."

"Gibbs."

"Doesn't ring a bell." Disappointment colored Mac's tone.

Don grinned. "Trust me, you'd remember him if you met him. I think everyone who's met Gibbs has had his name burned into their brain."

"Officer or-- Hang on a second." A soft thump came over the line, probably Mac covering the receiver, and a muffled conversation ensued. Don could only make out tone of voice, but Mac sounded a little impatient by the time he came back on the line. "Sorry, Don. As understaffed as this shift is, I probably shouldn't leave them on their own for long."

"I could call back," Don offered.

"No, it's okay. They should be able to take care of themselves for another half hour or so," Mac said. "We were about to get into Stefanos's client files, right?"

"Right." Don straightened up and blinked a few times to clear his eyes before he opened a drawer in his desk and pulled out a file folder. "I worked up a basic background on everyone on the list that you e-mailed me. You want to do this alphabetically or chronologically?"

"Chronologically," Mac said definitely. "Which means we're starting with HMD Incorporated..."