Or decimal expansion of electron Compton wavelength in meters.
Also decimal expansion of the relativistic de Broglie wavelength of an electron whose velocity is equal to c/sqrt(2), where c is the speed of light in vacuum in SI units. For more information see A229962.
The Compton wavelength is the wavelength of a photon whose energy is the same as the rest energy of an electron. - Omar E. Pol, Dec 03 2013
a(-3) is 3 or 4. - Ivan Panchenko, Jan 27 2015