222
1
+  = 24 + 1 = 17 is a prime 223
1
+  = 28 + 1 = 257 is a prime However, at first sight the generalisation looks to be correct But, eventually it was
shown that
225
1
+  = 232 + 1 = 4294967297
which is not a prime since 4294967297  =  641 × 6700417 (a product of two numbers)