For an elastic, isotropic collision in cm-system occurring with a heavy nucleus a) perform the limited expansion of ε and ξ in powers of 1/a; and b) evaluate the error in ξ when the exponential law for the gain in lethargy between 0 and ε is replaced by a uniform law between the same limits. this approximation is sometimes made, for instance, when calculating the strength of resonant absorption by heavy nuclides