maths.free › Abstract Algebra › Cyclic groups and permutation groups

Cyclic groups and permutation groups

Generators, orders, cycles, transpositions, parity.

A cyclic group is generated by one element; every permutation is a product of disjoint cycles, and its parity (even/odd) is well defined. Picture it: shuffling 5 cards as arrows between positions; a cycle is a loop of arrows. Think it: Cayley's theorem, every group is a permutation group, means permutations are the universal example.

Worked example · inverse of 5 mod 12

Number-Theory · Modular-Arithmetic

Inverse of 5 mod 12

5,\ 12

Step by step

  1. 5x \equiv 1 \pmod{12}

    We want x with a·x ≡ 1 (mod m). It exists only when gcd(a, m) = 1.

  2. 12 = 2 \times 5 + 2

    Euclid step.

  3. 5 = 2 \times 2 + 1

    Euclid step.

  4. 2 = 2 \times 1 + 0

    Euclid step.

  5. 5 \times 5 = 25 \equiv 1 \pmod{12}

    Back-substitute (extended Euclid) to express 1 as a combination of a and m; the coefficient of a is the inverse.

Reveal the answer
5^{-1} \equiv 5 \pmod{12}

Now you Pick a problem, or type or draw your own. Every step, a picture, the answer hidden until you ask.

Keep your own working

A free account adds notes on every lesson, a record of what you have finished, your solved problems in one place, and a tutor you can ask about this page. The maths itself is open to everyone, signed in or not.

Sign Up Login

Symbols used here

Tap any symbol for the full definition, a picture, and what every letter in it means.

How to: Cyclic groups and permutation groups

  1. We want x with a·x ≡ 1 (mod m). It exists only when gcd(a, m) = 1.
  2. Euclid step.
  3. Euclid step.
  4. Euclid step.
  5. Back-substitute (extended Euclid) to express 1 as a combination of a and m; the coefficient of a is the inverse.

Questions people ask

What is a group, in plain words?

A set with one operation that is associative, has an identity, and lets every element be undone. Symmetries of any object form a group. That is where the idea came from.

What is the difference between a ring and a field?

A ring has addition and multiplication that behave like the integers (you cannot always divide); a field is a ring where every non-zero element has a reciprocal, like the rationals or the reals.

More in Abstract Algebra

GroupsSubgroups, cosets and Lagrange's theoremHomomorphisms, normal subgroups and quotient groupsRings and fieldsGalois theory: why the quintic has no formula