# Clone

*of an operation*

Any set of finitary operations of the form $ \omega : A ^ {n} \rightarrow A $ that is closed with respect to composition and contains all the projections $ \omega _ {n} ^ {i} : A ^ {n} \rightarrow A $, defined by

$$ \omega _ {n} ^ {i} ( a _ {1} \dots a _ {i} \dots a _ {n} ) = a _ {i} $$

for any $ n $- tuple $ ( a _ {1} \dots a _ {n} ) $ in $ A ^ {n} $, where $ n \geq 1 $, $ i = 1 , 2 \dots $ and $ A $ is an arbitrary fixed set. By a composition of operations $ \omega _ {1} ( x _ {1} \dots x _ {j} \dots x _ {n} ) $ and $ \omega _ {2} ( y _ {1} \dots y _ {m} ) $ one means the operation $ \omega _ {3} ( z _ {1} \dots z _ {l} ) $ defined by the formula

$$ \omega _ {1} ( x _ {1} \dots x _ {j-} 1 ,\ \omega _ {2} ( y _ {1} \dots y _ {m} ) ,\ x _ {j+} 1 \dots x _ {n} ) $$

for some $ j \leq n $, where the sets of variables $ X = \{ x _ {1} \dots x _ {j} \dots x _ {n} \} $, $ Y = \{ y _ {1} \dots y _ {m} \} $ and $ Z = \{ z _ {1} \dots z _ {l} \} $ satisfy the equality

$$ Z = ( X \setminus \{ x _ {j} \} ) \cup Y ,\ \ m , l \geq 1. $$

#### Comments

The name "clone" was invented by P. Hall. It first appeared in print in (the first edition of) [a1].

#### References

[a1] | P.M. Cohn, "Universal algebra" , Reidel (1981) |

**How to Cite This Entry:**

Clone.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Clone&oldid=46362