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SUBROUTINE I0

PURPOSE
    COMPUTE THE MODIFIED BESSEL FUNCTION I OF ORDER ZERO

USAGE
    CALL I0(X,RI0)

DESCRIPTION OF PARAMETERS
    X	 -GIVEN ARGUMENT OF THE BESSEL FUNCTION I OF ORDER 0
    RI0  -RESULTANT VALUE OF THE BESSEL FUNCTION I OF ORDER 0

REMARKS
    LARGE VALUES OF THE ARGUMENT MAY CAUSE OVERFLOW IN THE
    BUILTIN EXP-FUNCTION

SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
   NONE

METHOD
   POLYNOMIAL APPROXIMATIONS GIVEN BY E.E. ALLEN ARE USED FOR
   CALCULATION.
   FOR REFERENCE SEE
   M. ABRAMOWITZ AND I.A. STEGUN,'HANDBOOK OF MATHEMATICAL
   FUNCTIONS', U.S. DEPARTMENT OF COMMERCE, NATIONAL BUREAU OF
   STANDARDS APPLIED MATHEMATICS SERIES, 1966, P.378.