series with coefficients given in the direct order, with binomials of odd order only

Hi all,
does anybody know how to create a polynomial series in the following
form: a[1]*x+a[2]*y+a[3]*x^3+a[4]*x^2*y
+a[5]*x*y^2+a[6]*y^3+a[7]*x^5+... I need a series with coefficients
given in the direct order, and odd order of binomials only...

Thanks,

Galina

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