# SLATEC Routines --- BESI ---

```*DECK BESI
SUBROUTINE BESI (X, ALPHA, KODE, N, Y, NZ)
C***BEGIN PROLOGUE  BESI
C***PURPOSE  Compute an N member sequence of I Bessel functions
C            I/SUB(ALPHA+K-1)/(X), K=1,...,N or scaled Bessel functions
C            EXP(-X)*I/SUB(ALPHA+K-1)/(X), K=1,...,N for non-negative
C            ALPHA and X.
C***LIBRARY   SLATEC
C***CATEGORY  C10B3
C***TYPE      SINGLE PRECISION (BESI-S, DBESI-D)
C***KEYWORDS  I BESSEL FUNCTION, SPECIAL FUNCTIONS
C***AUTHOR  Amos, D. E., (SNLA)
C           Daniel, S. L., (SNLA)
C***DESCRIPTION
C
C     Abstract
C         BESI computes an N member sequence of I Bessel functions
C         I/sub(ALPHA+K-1)/(X), K=1,...,N or scaled Bessel functions
C         EXP(-X)*I/sub(ALPHA+K-1)/(X), K=1,...,N for non-negative ALPHA
C         and X.  A combination of the power series, the asymptotic
C         expansion for X to infinity, and the uniform asymptotic
C         expansion for NU to infinity are applied over subdivisions of
C         the (NU,X) plane.  For values not covered by one of these
C         formulae, the order is incremented by an integer so that one
C         of these formulae apply.  Backward recursion is used to reduce
C         orders by integer values.  The asymptotic expansion for X to
C         infinity is used only when the entire sequence (specifically
C         the last member) lies within the region covered by the
C         expansion.  Leading terms of these expansions are used to test
C         for over or underflow where appropriate.  If a sequence is
C         requested and the last member would underflow, the result is
C         set to zero and the next lower order tried, etc., until a
C         member comes on scale or all are set to zero.  An overflow
C         cannot occur with scaling.
C
C     Description of Arguments
C
C         Input
C           X      - X .GE. 0.0E0
C           ALPHA  - order of first member of the sequence,
C                    ALPHA .GE. 0.0E0
C           KODE   - a parameter to indicate the scaling option
C                    KODE=1 returns
C                           Y(K)=        I/sub(ALPHA+K-1)/(X),
C                                K=1,...,N
C                    KODE=2 returns
C                           Y(K)=EXP(-X)*I/sub(ALPHA+K-1)/(X),
C                                K=1,...,N
C           N      - number of members in the sequence, N .GE. 1
C
C         Output
C           Y      - a vector whose first N components contain
C                    values for I/sub(ALPHA+K-1)/(X) or scaled
C                    values for EXP(-X)*I/sub(ALPHA+K-1)/(X),
C                    K=1,...,N depending on KODE
C           NZ     - number of components of Y set to zero due to
C                    underflow,
C                    NZ=0   , normal return, computation completed
C                    NZ .NE. 0, last NZ components of Y set to zero,
C                             Y(K)=0.0E0, K=N-NZ+1,...,N.
C
C     Error Conditions
C         Improper input arguments - a fatal error
C         Overflow with KODE=1 - a fatal error
C         Underflow - a non-fatal error (NZ .NE. 0)
C
C***REFERENCES  D. E. Amos, S. L. Daniel and M. K. Weston, CDC 6600
C                 subroutines IBESS and JBESS for Bessel functions
C                 I(NU,X) and J(NU,X), X .GE. 0, NU .GE. 0, ACM
C                 Transactions on Mathematical Software 3, (1977),
C                 pp. 76-92.
C               F. W. J. Olver, Tables of Bessel Functions of Moderate
C                 or Large Orders, NPL Mathematical Tables 6, Her
C                 Majesty's Stationery Office, London, 1962.
C***ROUTINES CALLED  ALNGAM, ASYIK, I1MACH, R1MACH, XERMSG
C***REVISION HISTORY  (YYMMDD)
C   750101  DATE WRITTEN
C   890531  Changed all specific intrinsics to generic.  (WRB)
C   890531  REVISION DATE from Version 3.2
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   900315  CALLs to XERROR changed to CALLs to XERMSG.  (THJ)
C   900326  Removed duplicate information from DESCRIPTION section.
C           (WRB)
C   920501  Reformatted the REFERENCES section.  (WRB)
C***END PROLOGUE  BESI
```