√-1 = i = i^5 = i^9 = i^13 = ...
-1 = i² = i^6 = i^10 = i^14 = ...
-i = i³ = i^7 = i^11 = i^15 = ...
1 =i^4 = i^8 = i^12 = i^16 = ...
So if the exponent is positive, divide the exponent by 4:
if the remainder is 0, the value is 1
if the remainder is 1, the value is i
if the remainder is 2, the value is -1
if the remainder is 3, the value is -i
For negative exponents:
i^-1 = 1/i x i/i = i/i² = i/-1 = -i = i^-5 = ...
i^-2 = 1/i² = 1/-1 = -1 = i^-6 = ...
i^-3 = i/i³ = i/-i = i = i^-7 = ...
i^-4 = 1/i^4 = i/1 = 1 = i^-8 = ...
So if the exponent is negative, divide the exponent by 4:
if the remainder is 0, the value is 1
if the remainder is 1, the value is -i
if the remainder is 2, the value is -1
if the remainder is 3, the value is i