Does Every Prime number (except $2,3,5$) has a multiple of form $10^k+1$ where $k\in N$?
I check till $29$ and it seems to be true. However it got gigantic. $29\times3448275862069=10^{14}+1$
I Proved using pigeonhole that every prime (except $2,5$) has multiple of form $10^k-1$ but am not able to do it with $10^k+1$
$$10^3-1=999=27\times 37.$$ So $10^3\equiv1\pmod {37}$. Modulo $37$ the powers of ten are $1,10,26,1,10,26,1,\ldots$. None of these are $\equiv-1\pmod{37}$.