“ The President shall from time to time award the medal, on the basis of recommendations received from the National Academy of Sciences or on the basis of such other information and evidence as lie deems appropriate, to individuals who in his judgment are deserving of special recognition by reason of their outstanding contributions to knowledge in the physical, biological, mathematical, or engineering sciences. ”

“ Not more than twenty individuals may be awarded the medal in any one calendar year. (c) An individual may not be awarded the medal unless at the time such award is made he— (1) is a citizen or other national of the United States ”