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Combinatorics Visual Tools

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Interactive explorers for every standard counting scenario: full, partial, circular and repetition permutations, combinations, partitions, distributions, weak and strong compositions, and the Pascal triangle — each one animated and adjustable so the formula and the arrangement it counts stay visibly connected.
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Combinatorics Formulas Reference

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Combinatorics formulas reference: factorial, permutation P(n,r), combination C(n,r), binomial coefficient, multinomial, stars and bars, Pascal
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Combinatorics Terms and Definitions

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Combinatorics glossary with 22 defined terms: counting principles, permutations, combinations, distributions, binomial coefficient, and Pascal
Counting Principles6
Addition RuleThe addition rule states that if a count splits into kk mutually exclusive cases with m1,m2,…,mkm_1, m_2, \ldots, m_k outcomes each, the total number of…Read more →Multiplication RuleThe multiplication rule states that if an outcome is built from kk independent steps with m1,m2,…,mkm_1, m_2, \ldots, m_k options each, the combined…Read more →Complementary CountingComplementary counting computes the size of a set by subtracting the size of its complement from the universe: if UU is the universe and…Read more →Double CountingDouble counting is a proof technique in which the same set is enumerated by two different strategies; the resulting expressions both equal the size…Read more →Pigeonhole PrincipleThe pigeonhole principle states that if nn items are distributed among kk containers and n>kn > k, then at least one container holds at least two…Read more →Inclusion-Exclusion PrincipleThe inclusion-exclusion principle computes the size of a union of nn sets by alternately adding the sizes of kk-fold intersections:…Read more →
FactorialThe factorial of a non-negative integer nn, written n!n!, is the product of all positive integers from 11 up to nn:…Read more →PermutationA permutation is an arrangement of objects in a definite order. The defining property is that order matters — different sequences of the same objects…Read more →Full PermutationA full permutation is an arrangement of all nn distinct items in a linear sequence, with each item appearing exactly once. The number of full…Read more →Partial PermutationA partial permutation is a selection of rr distinct items from nn available items followed by their arrangement into a linear sequence, with no…Read more →Permutation with RepetitionA permutation with repetition is an arrangement of rr positions where each position is filled independently from nn available items, with the same…Read more →Permutation with Identical ItemsA permutation with identical items is an arrangement of nn objects in a linear sequence where some objects are indistinguishable from one another,…Read more →Circular PermutationA circular permutation is an arrangement of nn distinct items around a circle, where two arrangements are considered identical if one is a rotation…Read more →DerangementA derangement is a permutation of a set in which no element appears in its original position. The number of derangements of an nn-element set,…Read more →
CombinationA combination is a selection of items from a collection where order does not matter. The number of ways to select rr items from nn distinct items…Read more →Partition into GroupsA partition into groups divides nn distinct items into kk unlabeled subsets where only the grouping matters, not the order within groups or names…Read more →Weak CompositionA weak composition is a distribution of nn identical items into rr labeled containers where some containers may remain empty. The number of weak…Read more →Strong CompositionA strong composition is a distribution of nn identical items into rr labeled containers where every container must receive at least one item. The…Read more →Distribution into CellsA distribution into cells assigns each of nn distinct items to one of rr labeled containers, producing a mapping from items to containers. The…Read more →
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Combinatorics Basics

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The complete counting toolkit on one page: the five counting principles, the inclusion-exclusion correction for overlapping sets, permutations versus combinations, the ten standard counting scenarios with their formulas, and the binomial coefficient with the binomial theorem — plus the bridges into probability, set theory, and algebra.
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Binomial Coefficient

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Binomial Theorem

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Binomial theorem for expanding (a+b)^n: general term formula, special cases, multinomial generalization, combinatorial proof, and worked example expansions.
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Combinatorics Calculator

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Calculate the number of ways to arrange nn distinct objects in a sequence.
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Combinations

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Combinations in combinatorics: simple combinations, partition into groups, weak and strong composition, and distribution into cells, with formulas and examples.
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step 1: shirt

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Counting principles of combinatorics: addition rule, multiplication rule, complementary counting, double counting, and the pigeonhole principle explained.
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French

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Inclusion-exclusion principle for counting unions of overlapping sets. Two-set, three-set, and general n-set formulas with derangement and surjection examples.
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Permutations

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Permutations in combinatorics: full, partial, with repetition, with identical items, circular, and derangements. Formulas, notation, and worked examples.
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