why 2,1-methylpropane,1,1-Dimethylpropane,1-methylpropane and several othres like this are not the isomers of pentane ? why has it only three isomers?

1- methylpropane is simply n-butane..so no isomer of pentane can be made...

1,1-dimethylpropane is also substituted butane....

2,1- methylpropane is wrongly wriiteen ... it should be 1,2-dimethyl propane and still it doent exist isomerically as it is again substituted butane and not pentane...