One theorem, principle, or idea from computer science per post: what it says, why the proof works, what it forbids forever, and – the part most writing skips – what it does not forbid.

Difficulty
IDResultKindDifficultyAreas
Part I Counting and Averaging Arguments
T001Pigeonhole principletheoremEasy#combinatorics#algorithms
T002Cantor's diagonal argumenttheoremEasy#logic#computability
T003The Omega(n log n) comparison-sort boundtheoremMedium#algorithms#combinatorics
T004No universal lossless compressortheoremEasy#information-theory#combinatorics
T005Adversary argumentstheoremMedium#algorithms#combinatorics
T007Linearity of expectationtheoremEasy#probability#algorithms
T008Concentration bounds (Chernoff/Hoeffding)theoremMedium#probability#algorithms
T074The birthday boundtheoremEasy#cryptography#probability#data-structures
Part II Self-Reference and the Limits of Computation
T010The Halting problemtheoremEasy#computability#logic
T011Rice's theoremtheoremMedium#computability#pl-theory#security
T012The Church–Turing thesisthesisEasy#computability
T013The universal machinetheoremEasy#computability#hardware
T014Kleene's recursion theoremtheoremMedium#computability#logic
T015Godel's incompleteness theoremstheoremMedium#logic
T017ReductionstheoremMedium#computability#complexity
Part III Information and Randomness
T018Shannon entropy and the source coding theoremtheoremMedium#information-theory
T020Huffman coding is optimaltheoremMedium#information-theory#algorithms
T021The noisy-channel coding theoremtheoremHard#information-theory#networking
T022The Hamming bound and error-correcting codestheoremMedium#information-theory#hardware
T023Kolmogorov complexitytheoremMedium#information-theory#computability#probability
T025Minimum description length and Occam's razortheorem principleMedium#information-theory#machine-learning
T106The Nyquist–Shannon sampling theoremtheoremMedium#information-theory#hardware
T110The no-cloning theoremphysical-lawMedium#hardware#information-theory#cryptography
Part IV Complexity: Hard, and Provably So
T026P vs NPopen-problemEasy#complexity
T027The Cook–Levin theoremtheoremMedium#complexity#logic
T028Karp's 21 problemstheoremMedium#complexity#algorithms
T030The barriers: relativization and natural proofstheoremHard#complexity#logic
T031The PCP theorem and inapproximabilitytheoremHard#complexity
T032Approximation algorithms and ratiostheoremMedium#complexity#algorithms#optimization
T033Parameterized complexity (FPT and W[1])theoremMedium#complexity#algorithms
T034Parity is not in AC^0theoremHard#complexity#hardware
T035BPP, pseudorandomness, and derandomizationtheoremHard#complexity#probability
T036IP = PSPACEtheoremHard#complexity#cryptography
T037The Exponential Time Hypothesis and fine-grained complexityconjectureMedium#complexity#algorithms
Part V Languages, Logic, and Types
T038The Chomsky hierarchytheoremEasy#automata#compilers
T039Kleene's theorem: regexes are finite automatatheoremMedium#automata#compilers
T040The Myhill–Nerode theoremtheoremMedium#automata
T041The lambda calculus and the Y combinatortheoremMedium#pl-theory#computability
T042The Curry–Howard correspondencetheoremMedium#type-theory#logic#pl-theory
T043Hindley–Milner and principal typestheoremMedium#type-theory#pl-theory
T044Progress and preservationtheoremMedium#type-theory#formal-methods
T045Parametricity: theorems for freetheoremMedium#type-theory#pl-theory
T047Abstract interpretation and Galois connectionstheoremHard#formal-methods#compilers#pl-theory
T048Hoare logic and weakest preconditionstheoremMedium#formal-methods#logic
T049Model checking, LTL, and Buchi automatatheoremHard#formal-methods#concurrency#automata
T107Register allocation is graph coloringtheoremMedium#compilers#complexity#algorithms
T050Resolution and CDCLtheorem empiricalMedium#logic#algorithms#formal-methods
Part VI Concurrency
T051The Coffman conditionstheoremEasy#concurrency#operating-systems
T109Work stealing and fair schedulingtheoremMedium#concurrency#performance#algorithms
T052Mutual exclusion without hardware supportdemonstration theoremMedium#concurrency#operating-systems
T053Herlihy's wait-free consensus hierarchytheoremHard#concurrency#hardware
T055LinearizabilitytheoremMedium#concurrency#distributed-systems
T056Memory models and sequential consistencytheoremMedium#concurrency#hardware#compilers
Part VII Distributed Systems
T058Happens-before and Lamport clockstheoremEasy#distributed-systems
T059Vector clocks and causal consistencytheoremMedium#distributed-systems
T061The Two Generals problemtheoremEasy#distributed-systems#networking
T062FLP impossibilitytheoremHard#distributed-systems
T063The Byzantine generals problemtheoremMedium#distributed-systems#security
T064Paxos, Raft, and the shape of consensusdemonstration theoremMedium#distributed-systems#databases
T065Quorum intersection (R + W > N)theoremEasy#distributed-systems#databases
T066The CAP theorem and PACELCtheoremMedium#distributed-systems#systems-design
T067The CALM theorem and CRDTstheoremHard#distributed-systems#pl-theory
Part VIII Data and Storage
T069Codd's theoremtheoremMedium#databases
T071Conflict serializability and two-phase lockingtheoremMedium#databases#concurrency
T009Balls into bins and the power of two choicestheoremMedium#probability#distributed-systems#performance
T104Universal hashingdemonstration theoremMedium#data-structures#probability#algorithms
T072Consistent hashingdemonstration theoremEasy#distributed-systems#data-structures
T073Bloom filtersdemonstration theoremEasy#data-structures#probability
T105Streaming lower bounds and sketchingtheoremMedium#information-theory#data-structures#algorithms
Part IX Security and Cryptography
T075Kerckhoffs's principle and perfect secrecyprinciple theoremMedium#cryptography#security
T076One-way functions and trapdoorsconjectureMedium#cryptography#complexity
T077Zero-knowledge proofsdemonstration theoremHard#cryptography#complexity
T078Merkle treesdemonstration theoremEasy#cryptography#data-structures
T079Reflections on Trusting TrustdemonstrationMedium#security#compilers
T080Saltzer and Schroeder's principlesprincipleEasy#security#systems-design
Part X Systems, Performance, and Practice
T081Amdahl's lawtheoremEasy#performance#concurrency
T082Gustafson's law and the Universal Scalability Lawempirical theoremMedium#performance#distributed-systems
T083Little's lawtheoremEasy#performance#systems-design
T084Latency under utilizationtheoremMedium#performance#systems-design
T108Token buckets, arrival curves, and network calculustheoremMedium#networking#performance#systems-design
T006Amortized analysis and the potential methodtheoremMedium#algorithms#data-structures
T086Competitive analysis and ski rentaltheoremEasy#algorithms#systems-design
T085Belady's optimal algorithm and LRU competitivenesstheoremMedium#operating-systems#algorithms#performance
T087The end-to-end argumentprincipleEasy#networking#systems-design
T088Memory hierarchy and localityempirical theoremMedium#performance#hardware#operating-systems#algorithms
T089Conway's lawempiricalEasy#software-engineering#systems-design
T090No Silver Bullet and Brooks's lawempiricalEasy#software-engineering
T091Hyrum's law and Postel's principleempirical principleEasy#software-engineering#networking#systems-design
Part XI Optimization and Mechanism Design
T092Max-flow min-cuttheoremMedium#optimization#algorithms
T093LP dualitytheoremMedium#optimization
T094Matroids and greedy optimalitytheoremHard#algorithms#optimization
T095Gale–Shapley stable matchingtheorem demonstrationEasy#optimization#algorithms
T101Arrow's impossibility theoremtheoremMedium#optimization#systems-design
Part XII Learning
T096PAC learning and VC dimensiontheoremHard#machine-learning
T097The bias–variance decompositiontheoremMedium#machine-learning
T098No Free LunchtheoremMedium#machine-learning
T099Why the generalization bounds are vacuousempirical theoremMedium#machine-learning
T100The Johnson–Lindenstrauss lemmatheoremMedium#machine-learning#probability#data-structures
Part XIII Physical Limits
T102Landauer's principlephysical-lawMedium#information-theory#hardware
T103Shor and Grover: what quantum actually buystheoremHard#hardware#cryptography#complexity