operation that is commutative but not associative
18817
Does commutativity imply Associativity?
151251
A simple algebra that is not semisimple
117793
Finding a function $\mathbb{N} \to \mathbb{N}$ that is surjective but not injective.
76547
Why are addition and multiplication commutative, but not exponentiation?
101512
Is there an numeric arithmetic with a single operator?
22245
Show the union of two subrings is generally not a subring
73056
Operation that makes the negative real numbers, $\mathbb{R}_{<0}$, a group
47577
Are Variable Operators Possible?
76749
Example of a finite non-commutative ring without a unity
46843
A ring without the Invariant Basis Number property
114830
bit-wise operation unary ~ (invert)
126771
Group element not taken to its inverse by any automorphism
108930
Show that an operation being commutative is a structural property
85819
A non-polyhedral pair $(X,A)$ with both $X$ and $A$ polyhedral
148731
Constructing a bijection between $[a,b)$ to $(0,1)$
127664
Inverse elements in the absence of identities/associativity.
114662
Construction of a specific non-commutative and infinite group (with conditions on the order of the elements)