Quote:
Originally Posted by Dr Sardonicus
...FWIW, I checked 10^n + 7 up to the limit n = 2000, and found pseudoprimes for the exponents
n = 1, 2, 4, 8, 9, 24, 60, 110, 134, 222, 412, 700, 999, and 1383.
I don't see any obvious pattern. ...

10^n+7 is in Kamada's collection. Many more PRPs are known for it, as well as many factorizations for n<=300