%N Numerator of Maclaurin expansion of (t*sqrt(t^2+1) + arcsinh(t))/2, the arc length of Archimedes' spiral.
%C From _Mikhail Gaichenkov_, Feb 05 2013: (Start)
%C For Archimedean spiral (r=at) and the arc length s(t)= a(t*sqrt(t^2+1) + arcsinh(t))/2, the limit of s’’(t)=a, t- -> infinity. In other words, a point moves with uniform acceleration along the spiral while the spiral corresponds to the locations over time of a point moving away from a fixed point with a constant speed along a line that rotates with constant angular velocity.
%C The error of approximation for large t: |a-s’’(t)| ~ a/(2(1+t^2)) (Gaichenkov private research).
%C The arc of the Archimedean spiral is approximated by the differential equation in polar coordinates r’^2+r^2=(at)^2 (see A202407). (End)
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ArchimedesSpiral.html">Archimedes' Spiral</a>
%e t + t^3/6 - t^5/40 + t^7/112 - (5*t^9)/1152 + (7*t^11)/2816 - ...
%Y Denominators are in A002595.
%A _Eric W. Weisstein_, Dec 22 2003