A \emph{hoop} is a structure $\mathbf{A}=\langle A,\cdot,\rightarrow,1\rangle $ of type $\langle 2,2,0\rangle$ such that
$\langle A,\cdot ,1\rangle $ is a commutative monoid
$x\rightarrow ( y\rightarrow z) = (x\cdot y)\rightarrow z$
$x\rightarrow x=1$
$(x\rightarrow y)\cdot x = (y\rightarrow x)\cdot y$
Remark: This definition shows that hoops form a variety.
Hoops are partially ordered by the relation $x\leq y \iff x\rightarrow y=1$.
The operation $x\wedge y = (x\rightarrow y)\cdot x$ is a meet with respect to this order.
A \emph{hoop} is a structure $\mathbf{A}=\langle A,\cdot,\rightarrow,1\rangle $ of type $\langle 2,2,0\rangle$ such that
$x\cdot y = y\cdot x$
$x\cdot 1 = x$
$x\rightarrow ( y\rightarrow z) = (x\cdot y)\rightarrow z$
$x\rightarrow x=1$
$(x\rightarrow y)\cdot x = (y\rightarrow x)\cdot y$
A \emph{hoop} is a structure $\mathbf{A}=\langle A,\cdot,\rightarrow,1\rangle $ of type $\langle 2,2,0\rangle$ such that
$\langle A,\cdot ,1\rangle $ is a commutative monoid
and if $x\le y$ is defined by $x\rightarrow y = 1$ then
$\le$ is a partial order,
$\rightarrow$ is the residual of $\cdot$, i.e., $\ x\cdot y\le z \iff y\le x\rightarrow z$, and
$(x\rightarrow y)\cdot x = (y\rightarrow x)\cdot y$.
Let $\mathbf{A}$ and $\mathbf{B}$ be hoops. A morphism from $\mathbf{A}$ to $\mathbf{B}$ is a function $h:A\rightarrow B$ that is a homomorphism:
$h(x\cdot y)=h(x)\cdot h(y)$, $h(x\rightarrow y)=h(x)\rightarrow h(y) $, $h(1)=1$
Example 1:
Finite hoops are the same as generalized BL-algebras (= divisible residuated lattices) since the join always exists in a finite meet-semilattice with top, and since all finite GBL-algebras are commutative and integral.
$\begin{array}{lr}
f(1)= &1
f(2)= &1
f(3)= &2
f(4)= &5
f(5)= &10
f(6)= &23
f(7)= &49
\end{array}$