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 A128867 Let f(i) = prime( f(i - 1) (modulo 10^n) ) with f(0) = 1; a(n) is the length of the period of the sequence f(i). 2
 4, 5, 31, 106, 53, 582, 318, 9528, 11201, 19174, 142177, 315394, 648675 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS EXAMPLE For a(1), the sequence is 1, 2, 3, 5, 11, 2, 3, 5, 11, 2, 3, 5, 11, ... The sequence has period {2, 3, 5, 11} so a(1) = 4. For a(2) see the A112279: 1, 2, 3, 5, 11, 31, 127, 103, 5, 11, 31, 127, 103, 5, 11, ..., . This sequence has a cyclic length of 5. MATHEMATICA f[n_] := Block[{k = 1, a}, a[0] = 1; a[i_] := a[i] = Prime[Mod[a[i - 1], 10^n]]; While[t = Table[a[i], {i, 0, k - 1}]; MemberQ[t, a[k]] == False, k++ ]; k + 1 - Flatten[ Position[ t, a[k]]][[1]]]; Array[ f, 10] CROSSREFS Sequence in context: A265709 A265708 A224219 * A262031 A326998 A270286 Adjacent sequences:  A128864 A128865 A128866 * A128868 A128869 A128870 KEYWORD nonn,more AUTHOR Robert G. Wilson v, Apr 05 2007 EXTENSIONS a(11) from Chai Wah Wu, Sep 26 2019 a(12)-a(13) from Chai Wah Wu, Oct 02 2019 a(11) and a(12) verified by Robert G. Wilson v, Oct 22 2019 STATUS approved

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Last modified June 23 13:01 EDT 2021. Contains 345401 sequences. (Running on oeis4.)