why latex subscript prints signs

hi i am using latex and texmaker to do the following:

开发者_运维问答

             $\mathcal{a( X, Y )= a_i \circ a_j}$

which i expect to get a(X,Y)= a subscript {i} circle a subscript {j}

but instead i get weird signs instead for a's (on the right side of equation), i and j ...can you tell me why? thanks...

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The problem is that \mathcal only works on upper case letters.

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never mind i did

          $a(X,Y)=a_i \circ a_j$

it solved the problem.

thanks anyways!