A338792 a(n) is the least number k such that 1/prime(k) has repeating decimal expansion of period n.
1, 2, 5, 12, 26, 13, 4, 52, 21, 28693, 1128, 2431, 1221, 16, 71954, 11, 7, 153888, 8, 27417323062119920, 496, 14, 9, 223378173194137397198, 5760923, 2403, 149, 134, 10, 452, 47, 406, 71, 19, 27, 20, 37607875619, 150886, 22544062111497849
Offset: 0
Examples
1/prime(1) = 1/2 = 0.5 (finite decimal expansion). 1/prime(2) = 1/3 = 0.3(3)... (period 1). 1/prime(5) = 1/11 = 0.09(09)... (period 2). 1/prime(12) = 1/37 = 0.027(027)... (period 3).