The deviation being the reciprocal of the corresponding power of phi:
a(n) - phi^n = 1/(-phi)^n, e.g.
n = 1; a(1) = 2; phi = 1.618..; 1/phi = 0.618.. -> 1 - phi^1 = -1/phi^1;
n = 2; a(2) = 3; phi^2 = 2.618..; 1/phi^2 = 0.382.. -> 3 - phi^2 = 1/phi^2;
n = 3; a(3) = 4; phi^3 = 4.236..; 1/phi^3 = 0.236.. -> 4 - phi^3 = -1/phi^3;
and - trivially - for n = 0; a(0) = 2 -> 2 - phi^0 = 1/phi^0).
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