Fortran Wigner 3-j, 6-j, 9-j functions

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This is a library of functions for computation of Wigner 3-j, 6-j and 9-j symbols using algebraic expressions in terms of factorials. It is expected to be accurate to $10^{-10}$ relative error for values less than about j=20.

List of real(kind=8) functions:

• logfac(n)
• logdoublefac(n)
• triangle(two_j1, two_j2, two_j3)
• vector_couple(two_j1, two_m1, two_j2, two_m2, two_jc, two_mc)
• threej(two_j1, two_j2, two_j3, two_m1, two_m2, two_m3)
• threej_lookup(two_j1,two_j2,two_j3,two_l1,two_l2,two_l3)
• sixj(two_j1,two_j2,two_j3,two_l1,two_l2,two_l3)
• sixj_lookup(two_j1,two_j2,two_j3,two_l1,two_l2,two_l3)
• ninej(two_j1,two_j2,two_j3,two_j4,two_j5,two_j6,two_j7,two_j8,two_j9)

List of subroutines:

• threej_table_init(min2j, max2j)
• sixj_table_init(min2j, max2j)

All integer arguments are 2j in order to accomadate half-integer arguments while taking advantage of faster integer-arithmetic. Invalid arguments return 0d0 and program continues.

Optionally, compile with OpenMP to accelerate table initialization.