FGH:
1.f_0(x) = x+1
2.f_n+1(x) = f_n(f_n(...f_n(x)...))(x个f_n)
3.f_n(x) = f_n[x](x)(n不为0或后继序数)
MGH:
1.m_0(x) = x+1
2.m_n+1(x) = m_n(m_n(x))
3.m_n(x) = m_n[x](x)(n不为0或后继序数)
HH:
1.H_0(x) = x
2.H_n+1(x) = H_n(x+1)
3.H_n(x) = H_n[x](x)(n不为0或后继序数)
SGH:
1.S_0(x) = 0
2.S_n+1(x) = S_n(x)+1
3.S_n(x) = S_n[x](x)(n不为0或后继序数)