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).

4, 5, 31, 106, 53, 582, 318, 9528, 11201, 19174, 142177, 315394, 648675

Offset

1,1

Links

Table of n, a(n) for n=1..13.

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