# Difference between revisions of "Tour:Every nontrivial subgroup of the group of integers is cyclic on its smallest element"

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## Latest revision as of 18:06, 8 September 2008

**This article adapts material from the main article:** every nontrivial subgroup of the group of integers is cyclic on its smallest element

This page is part of the Groupprops guided tour for beginners (Jump to beginning of tour)PREVIOUS: Equivalence of definitions of cyclic group|UP: Introduction four (beginners)|NEXT: Subgroup containment relation equals divisibility relation on generators

General instructions for the tour | Pedagogical notes for the tour | Pedagogical notes for this part

WHAT YOU NEED TO DO:

- Read the statement and understand it thoroughly.
- Go through the proof very carefully. Make sure you understand all the steps of the proof, particularly the application of Euclidean division.

## Statement

Let be a subgroup of , the group of integers under addition. Then, there are two possibilities:

- is the trivial subgroup, i.e.
- contains a smallest positive element, say , and is the set of multiples of . Thus, is an infinite cyclic group generated by , and is isomorphic to . We typically write .

## Proof

**Given**: A nontrivial subgroup of , the group of integers under addition

**To prove**: There exists a smallest positive element in , and , so is isomorphic to

**Proof**: First, observe that if is nontrivial, then there exists a nonzero element in . This element may be either positive or negative. However, since is a subgroup, it is closed under taking additive inverses, so even if the element originally picked was negative, we have found a positive number in .

Thus, the set of positive numbers in is nonempty. Hence, there exists a smallest positive number in . Call it .

Clearly, all the integer multiples of are in . We need to prove that *every* element in is a multiple of .

By the Euclidean division algorithm, we can write:

where are integers and . Since , . Thus, is a nonnegative integer less than such that . By the minimality of , we have , so , as desired.

Thus, , or is the set of multiples of .

An explicit isomorphism from to is given by the map sending an integer to the integer .

This page is part of the Groupprops guided tour for beginners. Make notes of any doubts, confusions or comments you have about this page before proceeding.PREVIOUS: Equivalence of definitions of cyclic group|UP: Introduction four (beginners)|