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c/* ********************************************************* */
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c/* Equivalent photon approximation structure function. * */
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c/* Improved Weizsaecker-Williams formula * */
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c/* V.M.Budnev et al., Phys.Rep. 15C (1975) 181 * */
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c/* ********************************************************* */
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c provided by Tomasz Pierzchala - UCL
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real*8 function epa_electron(x,q2max)
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data PI/3.14159265358979323846/
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data xin/0.511d-3/ !electron mass in GeV
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C // x = omega/E = (E-E')/E
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q2min= xin*xin*x*x/(1-x)
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if(q2min.lt.q2max) then
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& (2d0*xin*xin*x*(-1/q2min+1/q2max)+
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& (2-2d0*x+x*x)/x*dlog(q2max/q2min))
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c write (*,*) x,dsqrt(q2min),dsqrt(q2max),f
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real*8 function epa_proton(x,q2max)
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real*8 f, qmi,qma, q2max
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data PI/3.14159265358979323846/
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data xin/0.938/ ! proton mass in GeV
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C // x = omega/E = (E-E')/E
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qmi= xin*xin*x*x/(1-x)
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f = alpha/PI*(phi_f(x,q2max/qz)-phi_f(x,qmi/qz))*(1-x)/x
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real*8 function phi_f(x,qq)
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f=(1+a*y)*(-log(qq1/qq)+1/qq1+1/(2*qq1*qq1)+1/(3*qq1*qq1*qq1))
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f=f + (1-b)*y/(4*qq*qq1*qq1*qq1);
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f=f+ c*(1+y/4)*(log((qq1-b)/qq1)+b/qq1+b*b/(2*qq1*qq1)+
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$b*b*b/(3*qq1*qq1*qq1))