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Manuscript Title: Fortran program for a numerical solution of the nonsinglet Altarelli- Parisi equation.
Authors: R. Kobayashi, M. Konuma, S. Kumano
Program title: LAG2NS
Catalogue identifier: ADAV_v1_0
Distribution format: gz
Journal reference: Comput. Phys. Commun. 86(1995)264
Programming language: Fortran.
Computer: SUN-IPX.
Operating system: SUN-OS-4.1.2, VAX/VMS V5.5-2.
Keywords: Particle physics, Elementary, QCD, Altarelli-Parisi Equation, Numerical solution, Q**2 evolution, Laguerre polynomials.
Classification: 11.5.

Nature of problem:
This program solves the Altarelli-Parisi equation for a spin-independent flavour-nonsinglet structure function or quark distribution.

Solution method:
We expand an initial quark distribution (or a structure function) and a splitting function by Laguerre polynomials. Then, the solution of the Altarelli-Parisi equation is expressed in terms of the Laguerre expansion coefficients and the Laguerre polynomials.

This program is used for calculating Q**2 evolution of a spin- independent flavour-nonsinglet structure function or quark distribution in the leading order or in the next-to-leading-order of alphas. Double precision arithmetic is used. The renormalization scheme is the modified minimal subtraction scheme (MSbar). A user provides the initial structure function or the quark distribution as a subroutine. Examples are explained in sections 3.2, 4.13 and 4.14. Then, the user inputs thirteen parameters explained in section 3.1.

Running time:
Approximately five (three) seconds on SUN-IPX (VAX-4000/500) if the initial distribution is provided in the form of
    b1      c1     b2     c2                                             
 a x   (1-x)  + a x  (1-x)  + ...  
  1              2 
in the subroutine GETFQN. If Laguerre coefficients of the initial distribution in the subroutine FQNS(x) are calculated by a GAUSS quadrature, the running time becomes longer depending on the function form.