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A292411
a(n) = ((prime(n) - 1)/2)^2 modulo prime(n).
1
1, 4, 2, 3, 10, 13, 5, 6, 22, 8, 28, 31, 11, 12, 40, 15, 46, 17, 18, 55, 20, 21, 67, 73, 76, 26, 27, 82, 85, 32, 33, 103, 35, 112, 38, 118, 41, 42, 130, 45, 136, 48, 145, 148, 50, 53, 56, 57, 172, 175, 60, 181, 63, 193, 66, 202, 68, 208, 211, 71, 220, 77, 78, 235, 238, 83, 253, 87, 262
OFFSET
2,2
COMMENTS
Also the square of (prime(n) - 1)/2 + 1 modulo prime(n).
EXAMPLE
The fourth prime is 7; ((7 - 1)/2)^2 = 3^2 = 9 = 2 mod 7. Hence a(4) = 2.
The fifth prime is 11; ((11 - 1)/2)^2 = 5^2 = 25 = 3 mod 11. Hence a(5) = 3.
MATHEMATICA
Table[PowerMod[(Prime[n] - 1)/2, 2, Prime[n]], {n, 2, 70}]
PROG
(PARI) a(n) = lift(Mod((prime(n)-1)/2, prime(n))^2); \\ Michel Marcus, Sep 19 2017
CROSSREFS
Cf. A005097.
Sequence in context: A026177 A026196 A026216 * A069547 A282772 A227041
KEYWORD
nonn,easy
AUTHOR
Alonso del Arte, Sep 16 2017
STATUS
approved