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SUBROUTINE PERM(IPERM,NELS,NMOS,MAXMOS,NPERMS)
DIMENSION IPERM(MAXMOS,60), IADD(20), NEL(20)
************************************************************************
*
* PERM PERMUTES NELS ENTITIES AMONG NMOS LOCATIONS. THE ENTITIES AND
* LOCATIONS ARE EACH INDISTINGUISHABLE. THE PAULI EXCLUSION
* PRINCIPLE IS FOLLOWED. THE NUMBER OF STATES PRODUCED IS GIVEN
* BY NMOS!/(NELS!*(NMOS-NELS)!).
* ON INPUT: NELS = NUMBER OF INDISTINGUISHABLE ENTITIES
* NMOS = NUMBER OF INDISTINGUISHABLE LOCATIONS
*
* ON OUTPUT IPERM = ARRAY OF PERMUTATIONS, A 0 INDICATES NO ENTITY,
* A 1 INDICATES AN ENTITY.
* NPERM = NUMBER OF PERMUTATIONS.
*
************************************************************************
IF(NELS.GT.NMOS)THEN
WRITE(6,'('' NUMBER OF PARTICLES,'',I3,'' GREATER THAN NO. '',
1''OF STATES,'',I3)')NELS,NMOS
NPERMS=0
RETURN
ENDIF
NPERMS=1
DO 10 I=1,20
10 NEL(I)=1000
DO 20 I=1,NELS
20 NEL(I)=1
DO 50 I12=1-12+NELS,NMOS,NEL(12)
IADD(12)=I12
DO 50 I11=I12+1,NMOS,NEL(11)
IADD(11)=I11
DO 50 I10=I11+1,NMOS,NEL(10)
IADD(10)=I10
DO 50 I9=I10+1,NMOS,NEL(9)
IADD(9)=I9
DO 50 I8=I9+1,NMOS,NEL(8)
IADD(8)=I8
DO 50 I7=I8+1,NMOS,NEL(7)
IADD(7)=I7
DO 50 I6=I7+1,NMOS,NEL(6)
IADD(6)=I6
DO 50 I5=I6+1,NMOS,NEL(5)
IADD(5)=I5
DO 50 I4=I5+1,NMOS,NEL(4)
IADD(4)=I4
DO 50 I3=I4+1,NMOS,NEL(3)
IADD(3)=I3
DO 50 I2=I3+1,NMOS,NEL(2)
IADD(2)=I2
DO 50 I1=I2+1,NMOS,NEL(1)
IADD(1)=I1
DO 30 J=1,NMOS
30 IPERM(J,NPERMS)=0
DO 40 J=1,NELS
40 IPERM(IADD(J),NPERMS)=1
NPERMS=NPERMS+1
IF(NPERMS.GT.61)THEN
WRITE(6,'('' NUMBER OF PERM
1UTATIONS TOO GREAT, LIMIT 60'')')
GOTO 60
ENDIF
50 CONTINUE
60 NPERMS=NPERMS-1
RETURN
END
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