1 is its multiplicative identity.A left R-module M consists of an abelian group (''M'', +) and an operation '''·''' : ''R'' × ''M'' -> ''M'' such that for all r, s in R and x, y in M, we have
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The operation · is called scalar multiplication. Often the symbol · is omitted, but in this article we use it and reserve juxtaposition for multiplication in R. One may write _RM to emphasize that M is a left R-module. A right R-module M_R is