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User: Michael Sheridan

Michael Sheridan has authored 1 sequences.

A342204 Decimal expansion of the fixed point of the cosine function when measured in quadrants (1 quadrant = 90 degrees = Pi/2 radians).

5, 9, 4, 6, 1, 1, 6, 4, 4, 0, 5, 6, 8, 3, 5, 5, 8, 2, 9, 8, 8, 4, 6, 1, 8, 8, 4, 6, 3, 4, 7, 7, 3, 9, 2, 4, 7, 8, 9, 9, 4, 9, 3, 7, 2, 7, 1, 7, 0, 2, 5, 1, 4, 5, 0, 3, 0, 1, 8, 5, 7, 4, 3, 0, 1, 4, 2, 9, 9, 0, 2, 7, 9, 6, 3, 9, 3, 8, 5, 8, 5, 2, 6, 2, 9, 7, 7, 0, 6, 2, 0, 7, 5, 7, 8, 2, 8, 6, 5, 2

Offset: 0

Michael Sheridan, Mar 04 2021

This is analogous to A003957, the fixed point of the cosine function in radians and A330119, the fixed point of the cosine function in degrees.  Each of the three are the unique real solutions to cos(x)-x=0, in their respective angular units.  The quadrant unit offers a nice symmetry, cos(0)=1 and cos(1)=0.  Unlike the previous two, the quadrant fixed point is not an attractor of its cosine function.  It cannot be found by iterative cosine application. Although not proven, iterative quadrant cosine can be seen empirically to diverge for all initial values.
A graphical solution can be demonstrated by plotting y = cos(x*Pi/2) - x, which shows a single zero near x=0.6.
The bisection method converges for the entire range of the cosine function (-1 to 1).  Newton's method also converges with reasonable initial estimate.

			0.594611644056835582988461884634773924789949372717025145030185743014299027963938...

Cf. A003957, A330119.

RealDigits[x /. FindRoot[Cos[Pi*x/2] == x, {x, 1}, WorkingPrecision -> 105], 10, 100][[1]] (* Amiram Eldar, Mar 05 2021 *)

fpprec :100;
bf_find_root(cos(x*%pi/2)-x,x,0,1) ;

PARI
```
\p 100
solve(x=-1,1,cos(Pi*x/2)-x)
```

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User: Michael Sheridan

A342204 Decimal expansion of the fixed point of the cosine function when measured in quadrants (1 quadrant = 90 degrees = Pi/2 radians).

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