def eta(lam):
 if (lam > 0):
  return mp.sqrt((2 * (lam - mp.log((lam + 1)))))
 elif (lam < 0):
  return (- mp.sqrt((2 * (lam - mp.log((lam + 1))))))
 else:
  return 0

def eta ( lam ) : if ( lam > 0 ) : return mp . sqrt ( ( 2 * ( lam - mp . log ( ( lam + 1 ) ) ) ) ) elif ( lam < 0 ) : return ( - mp . sqrt ( ( 2 * ( lam - mp . log ( ( lam + 1 ) ) ) ) ) ) else : return 0
function from dlmf 8 .