A046272 Largest prime substring in 6^n (0 if none).

0, 0, 3, 2, 29, 7, 5, 7993, 67961, 769, 604661, 2797, 78233, 1306069, 783641, 7018498457, 2821109, 692665944473, 66841, 609359, 4400629, 21936950640377, 16217038422671, 3022305360281, 73838133832161689, 992970137, 170581728179