plotting Formulas with greek letters and variable values (expression, substitute, paste, ...)

text(0,7.5,substitute(expression(paste(symbol("t"),"(A) = ",c1 +
lg(A^c2),sep="")),list(c1=0,456,c2=-0.123)),pos=4)

It is much easier, just write the formula down in R syntax and 
substitute the relevant variables as in:

text(0, 7.5, substitute(tau(A) == a*lg(A^b), list(a=1.234, b=-0.567)))

Best,
Uwe Ligges