Felipe refers to my first ever paper (mathscinet.ams.org/mathscinet-getitem?mr=789713mathscinet.ams.org/mathscinet-getitem?mr=789713) from 1985 ! However I have a more recent paper that gives a better result along the lines asked for (mathscinet.ams.org/mathscinet-getitem?mr=2997580mathscinet.ams.org/mathscinet-getitem?mr=2997580) which shows that every 2^n-1$2^n-1$, with n<>1$n\ne1$ or 6$6$, has a primitive prime factor that divides it to an odd power.
Gerry Myerson
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