i think it is wrong.taking the lesser one as 1,t-1=1 t=2Tahmid Hasan wrote: 5.let $p^k+1=t^2$
let $p=2$,then $2^k=(t+1)(t-1)$
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if $p$ is odd then $p^k=(t+1)(t-1)$,$(t+1,t-1)=1$ since both are odd
then,$(p,k)=(3,1)$
i think it is wrong.taking the lesser one as 1,t-1=1 t=2Tahmid Hasan wrote: 5.let $p^k+1=t^2$
let $p=2$,then $2^k=(t+1)(t-1)$
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if $p$ is odd then $p^k=(t+1)(t-1)$,$(t+1,t-1)=1$ since both are odd