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paradox, accurate taylor series half iterate of eta not analytic at e
For base exp(1/e), there have been many posts that the half-iterate of is not analytic at z=e.

Below I am posting a paradoxical accurate 30 term Taylor series, for that non-analytic half iterate of , developed at z=e. This series is double precision accurate, out to a radius of around 1. An example, with z=e-0.6, and this series puts out 2.1507815747789682, which is the correct result for the half iterate, generated via .

The puzzle, is how is it possible to develop such a paradoxical accurate Taylor series, for a function which is not even analytic? A further complication, is that this Taylor series is required to seamlessly stitch together two different functions, the half iterate generated using , and the half iterate generated using the upper entire superfunction, . This is also the explanation for how it is possible. For a small enough series radius, it turns out the two half iterates can be seamlessly stitched together, with very little discontinuity, since as imag(z) increases, exponentially converges to . And the imag(z) stitching value gets larger as the Taylor series radius gets smaller. I sampled this series at a radius of 1.
. At imag(y)=4.9, the two functions half iterates are already consistent to an accuracy of approximately 15 digits, which allows for the merged Taylor series. For real(z)<e, we use the half iterate generated from , and for real(z)>=e, we use the half iterate generated from . The singularity and misbehavior occurs for the half iterate of cheta(z), when real(z)<=e, and similar misbehavior occurs for the half iterate of for real(z)>=e. At e itself, both functions have singularities, but both functions agree that the half iterate of e=e.

For real(z)=e, at smaller values of imag(z), paradoxically, the stitch is occurring for larger values of imag(y), where the stitch becomes more and more seamless. For the half iterate of , where the two functions are consistent to 31 decimal digits! But at a larger sampling radius of r=1.5, the two half iterates are only consistent to approximately 11 decimal digits. So, within acceptable accuracy limits, it turns out it is possible to develop a Taylor series for the half iterate of , at e, where the function is not even analytic.

a0=   2.7182818284590452
a1=   1.0000000000000000
a2=   0.091969860292860588
a3=   0.0028194850674294418
a4=  -8.0085798390301461 E-18
a5=   0.0000047696976272632850
a6=  -0.00000051177982848104645
a7=   0.0000000038423117936633581
a8=   0.000000014046730882359691
a9=  -0.0000000030441687473700918
a10= -1.0220786581293779 E-11
a11=  1.6377605039633389 E-10
a12= -2.8311894064410302 E-11
a13= -8.1773414171527632 E-12
a14=  4.0125808678932662 E-12
a15=  3.0797537350253160 E-13
a16= -5.8574107605901771 E-13
a17=  3.8771237610746716 E-14
a18=  1.0016443539438077 E-13
a19= -2.2477472152925032 E-14
a20= -2.0211321002183050 E-14
a21=  8.7613198763649902 E-15
a22=  4.7138857780993590 E-15
a23= -3.4998961878228463 E-15
a24= -1.2239509050193108 E-15
a25=  1.5383321198806298 E-15
a26=  3.3098494797748605 E-16
a27= -7.5476712927521906 E-16
a28= -7.8877856873676488 E-17
a29=  4.1188557816475547 E-16
a30=  3.5020724883949410 E-18

Messages In This Thread
paradox, accurate taylor series half iterate of eta not analytic at e - by sheldonison - 06/02/2011, 04:58 PM

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