## Statistical Mechanics and Mathematical Immersions

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When they'd first gotten together, Steve had needed a few weeks to get used to the idea of having a boyfriend. He'd grown up stifling these urges, and giving them free rein just felt wrong for a while.

It didn't take long, though, for Steve to figure out that Tony really liked math.

He'd had his suspicions by the second week, when he'd walked into the workshop to hear JARVIS reminding Tony yet again that normal people didn't get turned on by math. He'd always assumed that was an inside joke, but Tony's half-hard state, which he'd tried to hide by ignoring it in favor of kissing Steve, had said otherwise. Tony could try all he liked to pass off his state as a reaction to the kiss. Steve knew better.

After that, he'd picked up a book. Or, more accurately, asked JARVIS to summarize a few relevant courses.

It took him almost as much time to work through the maths as it did for him to get to the point of being comfortable with making out with Tony. But that had meant he'd had time to figure out how to approach things, and it seemed like it might be a good way to get Tony to move past kissing and to more interesting things. Things Steve had been curious about for something like a month now.

Tony had told him about what was entailed when they got together. Had asked JARVIS to gather some relevant information and give it to Steve. But they hadn't moved past the theory, and Steve kind of wanted to. Just reading about it was enough to get him worked up enough to ache for more, and his hand was slowly but surely getting unsatisfying.

Sometime soon, he resolved, he would get Tony to take things further.

Two and two always made four, after all. Steve smirked. Getting to the desired result was just a matter of how you framed the question.

He just needed to wait for the right moment.

He got it three days later. Tony was in the workshop, and Steve had followed him there. Sitting on the sofa in the corner that he suspected Tony used as a makeshift bed when he felt like it, Steve passed the time sketching for the first hour. Nothing wanted to quite turn out right, though, and his attention wandered to what he'd been reading.

His memory was all but eidetic, and his ability to parse things had only sharpened after the Serum. And Tony knew that. But that didn't mean he couldn't pretend to want help, for his opening gambit.

"Hey Tony," he started, "got a moment?"

"For you," Tony joked, finishing up what he was doing and turning to face him, "always. What do you need?"

There were two ways he could open the discussion. He chose the more suggestive one. "I've been trying to catch up on the things I missed, and I came across something that I didn't really understand, and it seems like a topic you'd know about."

Tony raised an eyebrow, clearly expecting some pop culture reference. "And what would that be," he asked, tone dry and a hint of a grimace on his face.

"Well, JARVIS showed me this thing be called Boy's Surface[wikipedia link]."

Tony gestured to the nearest holographic interface without a word, and JARVIS obediently displayed a three dimensional wireform of it. "What am I looking at, here, J," he prompted.

"Boy's surface is an immersion of the real projective plane in 3-dimensional space. It was discovered by Werner Boy in 1901, while on assignment from David Hilbert to prove that the projective plane could not be immersed in 3-space. Unlike other immersions satisfying the same conditions, it has no singularities, but it does self-intersect," JARVIS summarised.

Tony turned to stare at Steve, his expression somewhere between stunned and aroused.

Steve stared back.

It took Tony several long seconds to find his voice. He had to clear his throat twice before he could get the words out. "What did you want to know, Cap?"

"What the hell it is?"

Tony laughed, the sound tinged with dark promise, somehow. "It's pretty abstract, so I'm not surprised it gave you trouble. There are mathematical ways to turn a sphere inside out[wikipedia link]. This thing you've found is halfway through one of them[wikipedia link]."

Finished speaking, Tony stalked over to him, watching him carefully as he did, then draped himself over Steve. This level of contact was safe; they'd done more making out, really. "Hi," he grinned, straddling Steve's thighs and shamelessly showing off the toned muscle of his torso.

Steve grinned back, helpless in the face of Tony's direct charm. "Hi."

"Was that all you wanted to know," Tony asked, "'cause I'm not opposed to talking math some more."

It was the perfect opening. "Well," Steve ducked his head slightly, pretending embarrassment, but his confusion here really was genuine, "I'm still not sure what that stuff JARVIS said meant? What exactly is an immersion[wikipedia link]?"

Tony looked even more intrigued, and Steve enjoyed the way his eyes went dark, but Tony patted the front of his shoulder rather than immediately launch into a lecture. "That's gonna take a while to explain, and I'm assuming you have a few more questions. What else you got on your list? Got anything less complex?"

Steve couldn't help but notice that Tony was getting hard by the bulge in his jeans. He swallowed back a smirk, and replied, "I do, actually. Different topic, though."

"Oh?" Tony leaned in and threaded his hands through Steve's hair. "Well, don't keep me in suspense."

Raising his head to meet Tony's eyes properly again, Steve let his hands settle on Tony's hips, caressing the strip of bare skin between jeans and t-shirt with his thumbs. "I think I've more or less understood this one," he said, pulling Tony closer until they were chest-to-chest, "but I wanted to talk through it with an expert."

"Mmm," Tony pulled back just enough to catch his eyes and smirk at him. "And that means seducing said expert?"

Steve laughed at him. "You started it, coming over here to drape yourself over me."

"You tell yourself that. You totally started it, coming in here and talking math at me," Tony muttered absently, half under his breath. Tony knew Steve would hear him, but obviously wasn't thinking about that.

"If I'd known math was an appropriate seduction tactic, I might have tried it sooner," he replied, and saw the moment Tony realised what he'd said. And what Steve had said.

Steve didn't let him speak, pulling him in for a kiss and forcing the words back. It took them a few minutes to get tired of making out. When Tony pulled back far enough to speak, Steve pulled him in closer by his hips, until they were grinding against one another with movements made deliciously harsh by the fabrics of their clothes.

Voice slightly breathless now, Tony prompted him to continue, obviously quite happy to accept Steve's plan and go with it. "So tell me. What was it you wanted to talk about?"

Steve grinned. "Physics."

"Go on," Tony licked at his lover's lips and let his hands wander up Steve's arms to his broad shoulders. "Tell me more. What kind of physics?"

"Statistical mechanics," Steve told him, running his own hands up Tony's flanks then down his back to grab his ass. Tony jerked against him and moaned, voice low enough not to carry far, but wanton enough to send a jolt through Steve in response.

"Goddamn; that is just about the hottest thing you could have said," Tony said, sounding like he half wanted it to be a complaint, but was enjoying this far too much to actually make it one. "Keep talking. Details," he demanded, shoving at Steve's shoulders until he got the message and rearranged them on the sofa so that he was on his back, with Tony perched high on his thighs, and his head half propped against an armrest.

It was Steve's turn to clear his throat before he could speak, the sight of Tony leaning over him like that leaving his mouth dry and sending words scattering like pigeons. "Well, I haven't gotten very far yet," he said, watching avidly as Tony's hands ran down the length of his own torso to undo the button and fly of his 'shop jeans. "What I've read is still on the topic of ideal systems and non-interacting particles." As he finished the sentence, Tony tore off his t-shirt and started scrambling in the top drawer of the small work-table within his reach. When Tony straightened and turned back to him holding a condom and lube, Steve couldn't help the way he stopped speaking and just stared for a moment.

Tony peeled his underwear off as well, then, and started working Steve's clothes off. That was enough of a distraction that Tony had to prompt him to continue talking once they were both naked and pressed tightly together again. "So which model is it? There are a few that fit that description."

"Maxwell-Boltzmann statistics[wikipedia link]," Steve managed to say, as Tony took him in hand, rolling the condom onto him and working it until the drop of lube in the tip had coated him and made him feel almost more sensitive than he had, bare. It made his hips jerk without his conscious decision, wanting more of the sensation.

"If you have objections to me riding you right here and now, you'd better speak up now," Tony told him, a promising note in his voice under the slight roughness of his lust, "but you're gonna keep talking."

"About that sexy math and science," Tony replied, pouring out some more lube and reaching around behind himself.

Steve belatedly realised that Tony was preparing himself, and reached up to touch, moving quickly enough that Tony couldn't stop him. It got a good reaction, though. Tony's hard cock jerked and he made the most amazing needy sound when Steve's fingers touched his ass. It sent another shot of lust through Steve, and he kept up the touches that covered his hand in lube as he tried to refocus his thoughts enough to keep their discussion going. "If I understood it right," he eventually said, as he sank a fingertip into Tony alongside the fingers his lover already had there. The feeling of soft warm skin almost stopped him from finishing the sentence. "It's meant to describe the average distribution of energy states."

Steve made a protesting sound as Tony pulled out his own fingers and Steve's as well. He decided he wanted to do that part next time. The prospect of feeling that soft warmth wrapped around him was more than enough to keep him from grabbing for Tony to push his fingers back in, and deeper than he'd been able to while Tony'd had his own fingers there.

"Right so far," Tony answered, more breathless than before. "But there's more to it. What about the rest?"

It was getting difficult to think about that in the face of what Tony was doing. "The rest?"

"Yeah," Tony grinned at him. "What about the impact of quantum effects?"

Steve thought back. "Didn't come up much," he replied, breaking off to watch as Tony lined them up and slowly dropped his hips with a drawn-out pleased moan. Steve's voice caught and he forced the next sentence out through clenched teeth as he found out exactly what it felt like to be sheathed inside Tony. "That model only applies when those are negligible."

Tony stopped asking questions, then, in favor of saving his breath for moving. His thrusts were sinuous movements that started at his knees and ended at his shoulders. "Damn," he groaned, "this isn't gonna last long. You feel amazing, and I can't believe how hot it is when you talk science."

Gathering his wits, Steve went on; this had clearly been a great plan. "It doesn't work well at low temperatures, either," he added, the phrase broken by the way his breath hitched, seeming to get caught in his lungs and then again a few more times before hit his mouth and open air. "Because," he paused to take another breath, his head falling back onto the armrest of the sofa again, "that's when the quantum effects," Tony's reaction to that phrase made him gasp, the way his muscles tightened down around Steve sending shocks up both their spines, "start becoming important."

Tony whined, high in his throat and his movements got wilder, sending all the science he'd learned right back out of Steve's head. His hands clenched around the sofa cushions as he met Tony's thrusts with his own. It wouldn't take much more to send him over the edge either, at this point.

When Tony's hands landed on his abs and slid up to tweak at his nipples, Steve's teeth clenched with the effort of holding back, and he shifted his own hands up to settle on Tony, one behind Tony's left knee, and the other on his ass, guiding his movements and urging him onward.

It was just this side of too much, and Steve knew that if he wanted Tony to come first, he'd have to do something. He wasn't sure where the words came from, but he was fairly sure it would turn the trick. "The use of the Maxwell-Boltzmann model," he had to stop to force down the keening whine Tony's movements were pulling out of him, "also assumes that the individual -- oh! -- particles are distinguishable," he recited, one of the conditions for the use of the model being the first thing that came to mind.

Tony groaned, and came hard, his hands jerking away from where they'd been lying against Steve's skin. He reached down with one hand and took himself in hand, drawing out his climax, his muscles cording and his head thrown back. His other hand landed on Steve's hip for balance, holding tightly enough that it might bruise for a little while. The sight, combined with the way Tony suddenly tightened around him, sent Steve right over the edge with him. The world narrowed down to Tony and the way his heart pounded for a long moment.

As Tony relaxed, he slumped down against Steve, not bothering to disengage.

It took them a while to move, and when they did, it was reluctantly.

Tony whined at him, petulant, when Steve used his strength to lift him up bodily, off his softening cock. "I was enjoying that," he grumbled.

Steve huffed at him, amused. "That's good to know, but I'd like to get cleaned up."

"Get your ass back here when you're done," came the predictable demand, and Steve grinned. Tony liked to cuddle, and Steve had nothing against that whatsoever.

Grabbing his pants off the floor, Steve hurried to the small bathroom Tony had adjoining the workshop, and stripped off the condom that felt disgusting, now.

When he felt clean enough to pull his pants back on, he moistened a towel and carried it back with him. Tony, sprawled on the sofa on his back, hadn't bothered to do so much as clean his hand off on his t-shirt. Steve shook his head, and settled on the edge of the sofa to start cleaning him up.

Tony flinched at the touch of the cool towel on his still-heated skin, and batted lazily at Steve's hand. Snickering at his boyfriend, Steve caught the hand and kept working. When he was satisfied Tony was clean, he dropped the towel to the floor, unconcerned about the concrete, and stretched out next to him, pulling him close.

After a short silence, Tony spoke. "So you've found out about that little kink of mine. Got any of your own that you want to explore?"