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laps.for
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1985-11-29
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58 lines
C
C ..................................................................
C
C SUBROUTINE LAPS
C
C PURPOSE
C COMPUTES THE VALUE OF AN N-TERM EXPANSION IN LAGUERRE
C POLYNOMIALS WITH COEFFICIENT VECTOR C FOR ARGUMENT VALUE X.
C
C USAGE
C CALL LAPS(Y,X,C,N)
C
C DESCRIPTION OF PARAMETERS
C Y - RESULT VALUE
C X - ARGUMENT VALUE
C C - COEFFICIENT VECTOR OF GIVEN EXPANSION
C COEFFICIENTS ARE ORDERED FROM LOW TO HIGH
C N - DIMENSION OF COEFFICIENT VECTOR C
C
C REMARKS
C OPERATION IS BYPASSED IN CASE N LESS THAN 1
C
C SUBROUTINES AND FUNCTION SUBPROGRAMS REQUIRED
C NONE
C
C METHOD
C DEFINITION
C Y=SUM(C(I)*L(I-1,X), SUMMED OVER I FROM 1 TO N).
C EVALUATION IS DONE BY MEANS OF UPWARD RECURSION
C USING THE RECURRENCE EQUATION FOR LAGUERRE POLYNOMIALS
C L(N+1,X)=2*L(N,X)-L(N-1,X)-((1+X)*L(N,X)-L(N-1,X))/(N+1).
C
C ..................................................................
C
SUBROUTINE LAPS(Y,X,C,N)
C
DIMENSION C(1)
C
C TEST OF DIMENSION
IF(N)1,1,2
1 RETURN
C
2 Y=C(1)
IF(N-2)1,3,3
C
C INITIALIZATION
3 H0=1.
H1=1.-X
T=1.+X
C
DO 4 I=2,N
H2=H1-H0+H1-(T*H1-H0)/FLOAT(I)
H0=H1
H1=H2
4 Y=Y+C(I)*H0
RETURN
END