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A240665
Least k such that 10^k == -1 (mod prime(n)), or 0 if no such k exists.
2
0, 0, 0, 3, 1, 3, 8, 9, 11, 14, 0, 0, 0, 0, 23, 0, 29, 30, 0, 0, 4, 0, 0, 22, 48, 2, 17, 0, 54, 56, 21, 65, 4, 23, 74, 0, 39, 0, 83, 0, 89, 90, 0, 96, 49, 0, 15, 111, 0, 114, 116, 0, 15, 25, 128, 131, 134, 0, 0, 14, 0, 73, 0, 0, 156, 0, 55, 168, 0, 58, 16, 0
OFFSET
1,4
COMMENTS
The least k, if it exists, such that prime(n) divides 10^k + 1.
FORMULA
a(n) = A002371(n)/2 if A002371(n) is even, otherwise 0.
a(n) = A068958(n) for n > 3. - Georg Fischer, Oct 23 2018
MATHEMATICA
Table[p = Prime[n]; s = Select[Range[p/2], PowerMod[10, #, p] == p - 1 &, 1]; If[s == {}, 0, s[[1]]], {n, 100}]
CROSSREFS
Cf. A002371 (order of 10 mod prime(n)), A068958.
Sequence in context: A276228 A188938 A156368 * A068958 A238106 A087000
KEYWORD
nonn
AUTHOR
T. D. Noe, Apr 14 2014
STATUS
approved