Let $G = G(\cdot)$ be a commutative groupoid such that $\{(a,b,c) \in G^3$; $a\cdot bc \ne ab\cdot c\} = \{(a,b,c) \in G^3$; $a=b\ne c$ or $ a \ne b =c \}$. Then $G$ is determined uniquely up to isomorphism and if it is finite, then $\operatorname{card}(G) = 2^i$ for an integer $i\ge 0$.
@article{118571,
author = {Ale\v s Dr\'apal},
title = {On a class of commutative groupoids determined by their associativity triples},
journal = {Commentationes Mathematicae Universitatis Carolinae},
volume = {34},
year = {1993},
pages = {199-201},
zbl = {0787.20040},
mrnumber = {1241727},
language = {en},
url = {http://dml.mathdoc.fr/item/118571}
}
Drápal, Aleš. On a class of commutative groupoids determined by their associativity triples. Commentationes Mathematicae Universitatis Carolinae, Tome 34 (1993) pp. 199-201. http://gdmltest.u-ga.fr/item/118571/
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