---
name: mfe-foundations
description: "Pure mathematical structure. Sets, groups, rings, fields, topology — the formal bedrock everything else rests on."
user-invocable: false
allowed-tools: Read Grep Glob
metadata:
extensions:
gsd-skill-creator:
version: 1
createdAt: "2026-02-26"
triggers:
intents:
- "set"
- "logic"
- "proof"
- "group"
- "ring"
- "field"
- "topology"
- "axiom"
- "formal"
- "abstract"
contexts:
- "mathematical problem solving"
- "math reasoning"
---
# Foundations
## Summary
**Foundations** (Part VI: Defining)
Chapters: 18, 19, 20, 21
Plane Position: (-0.6, 0.6) radius 0.35
Primitives: 55
Pure mathematical structure. Sets, groups, rings, fields, topology — the formal bedrock everything else rests on.
**Key Concepts:** Set Definition (ZFC), Topological Space, Group Definition and Axioms, Propositional Logic (Boolean Operations), Predicate Logic (Quantifiers)
## Key Primitives
**Set Definition (ZFC)** (axiom): A set is a well-defined collection of distinct objects (elements). Membership is denoted x in S. Two sets are equal iff they have exactly the same elements (Axiom of Extensionality). Sets are the foundational objects of mathematics under ZFC.
- Define a collection of mathematical objects
- Establish the foundational objects for building mathematical structures
- Work with membership, inclusion, and equality of collections
**Topological Space** (axiom): A topological space (X, tau) is a set X with a collection tau of subsets (called open sets) satisfying: (1) emptyset and X are in tau. (2) Any union of sets in tau is in tau. (3) Any finite intersection of sets in tau is in tau.
- Define the concept of 'nearness' or 'openness' without a metric
- Study properties preserved under continuous deformation
- Generalize analysis to abstract settings
**Group Definition and Axioms** (axiom): A group (G, *) is a set G with a binary operation * satisfying: (1) Closure: a*b in G for all a,b in G. (2) Associativity: (a*b)*c = a*(b*c). (3) Identity: exists e in G such that e*a = a*e = a. (4) Inverses: for each a, exists a^{-1} with a*a^{-1} = a^{-1}*a = e.
- Verify if a set with an operation forms a group
- Identify symmetries of objects as group elements
- Study algebraic structures with a single binary operation
**Propositional Logic (Boolean Operations)** (definition): Propositional logic deals with propositions (true/false statements) combined by logical connectives: AND (conjunction, p ^ q), OR (disjunction, p v q), NOT (negation, ~p), IMPLIES (conditional, p -> q), IFF (biconditional, p <-> q).
- Combine simple statements into complex logical expressions
- Determine the truth value of a compound proposition
- Formalize arguments and reasoning
**Predicate Logic (Quantifiers)** (definition): Predicate logic extends propositional logic with variables, predicates P(x), and quantifiers: universal (forall x, P(x)) meaning P holds for all x, and existential (exists x, P(x)) meaning P holds for some x. Negation: ~(forall x, P(x)) iff (exists x, ~P(x)).
- Express mathematical statements involving 'for all' or 'there exists'
- Negate quantified statements correctly
- Formalize mathematical definitions and theorems
**Homomorphism** (definition): A group homomorphism f: G -> H is a function satisfying f(a *_G b) = f(a) *_H f(b) for all a, b in G. It preserves the group operation. The kernel ker(f) = {a in G : f(a) = e_H} is a normal subgroup of G. The image im(f) is a subgroup of H.
- Define a structure-preserving map between groups
- Identify the kernel and image of a group map
- Classify groups up to homomorphic relationships
**Open Set and Closed Set** (definition): In a topological space (X, tau), a set U is open if U in tau. A set C is closed if X \ C is open. The closure cl(A) is the smallest closed set containing A. The interior int(A) is the largest open set contained in A. A set can be both open and closed (clopen).
- Determine if a set is open, closed, or neither in a given topology
- Compute the closure, interior, and boundary of a set
- Work with topological properties defined via open/closed sets
**Cartesian Product** (definition): The Cartesian product of A and B is A x B = {(a,b) : a in A, b in B}. For n sets: A_1 x ... x A_n = {(a_1,...,a_n) : a_i in A_i}. |A x B| = |A| * |B|. R^n = R x R x ... x R (n times).
- Form all possible pairs from two sets
- Construct the domain for relations and functions
- Build multi-dimensional spaces from one-dimensional sets
**Relation** (definition): A relation R from A to B is a subset of A x B. We write aRb or (a,b) in R. Properties: reflexive (aRa), symmetric (aRb => bRa), antisymmetric (aRb and bRa => a=b), transitive (aRb and bRc => aRc).
- Define a relationship between elements of two sets
- Check if a relation has special properties (reflexive, symmetric, transitive)
- Formalize order, equivalence, or other structural relationships
**Equivalence Relation** (definition): An equivalence relation ~ on set A is a relation that is reflexive (a ~ a), symmetric (a ~ b => b ~ a), and transitive (a ~ b and b ~ c => a ~ c). It partitions A into disjoint equivalence classes [a] = {x in A : x ~ a}.
- Classify elements into groups where they are considered equivalent
- Partition a set into disjoint equivalence classes
- Define modular arithmetic or congruence relations
## Composition Patterns
- Set Definition (ZFC) + foundations-propositional-logic -> Set builder notation: {x in S : P(x)} uses logical predicates to define sets (parallel)
- Empty Set + foundations-set-definition -> Basis for inductive set construction: {}, {{}}, {{},{{}}}, ... (sequential)
- Set Union + foundations-set-intersection -> Boolean algebra of sets: union and intersection with complement form a complete Boolean algebra (parallel)
- Set Intersection + foundations-set-union -> Set algebra with distributive laws: A inter (B union C) = (A inter B) union (A inter C) (parallel)
- Set Complement + foundations-set-union -> De Morgan's laws for sets: (A union B)^c = A^c inter B^c and (A inter B)^c = A^c union B^c (parallel)
- Cartesian Product + foundations-relation -> Relations as subsets of Cartesian products: R subset A x B (sequential)
- Power Set + foundations-cardinality -> Cantor's theorem: |P(A)| > |A| for any set A, proving no largest cardinal (sequential)
- Relation + foundations-set-definition -> Relations as structured subsets of Cartesian products, enabling order theory (sequential)
- Equivalence Relation + foundations-group-definition -> Quotient group: G/N uses equivalence classes (cosets) as group elements (sequential)
- Equivalence Class / Partition Theorem + foundations-equivalence-relation -> Bijection between equivalence relations on A and partitions of A (parallel)
## Cross-Domain Links
- **structure**: Compatible domain for composition and cross-referencing
- **reality**: Compatible domain for composition and cross-referencing
- **mapping**: Compatible domain for composition and cross-referencing
- **unification**: Compatible domain for composition and cross-referencing
- **synthesis**: Compatible domain for composition and cross-referencing
## Activation Patterns
- set
- logic
- proof
- group
- ring
- field
- topology
- axiom
- formal
- abstract