[R] Relational Operators in Legend

[Lorenzo Isella]

Dear All,
I am sure that what I am asking can be solved by less than a
one-liner, but I am having a hard time to get this right.
Inside a legend environment, I do not have any problem if I issue the command:
expression(""*R[g]*"~"* k^{1/d[f]^{small}}*",  "*k<= {} *"15")
but I would like to write that 5<=k<=15. It has to be trivial, but so
far attempts like:
expression(""*R[g]*"~"* k^{1/d[f]^{small}}*", for  5 "*<={}*" "*k<= {} *"15")
always return some error message and no improvement with all the
variations I tried out.
Does anyone know what I am doing wrong?
Many Thanks

Lorenzo