Exact properties of Frobenius numbers and fraction of the symmetric   semigroups in the weak limit for n=3
  We generalize and prove a hypothesis by V. Arnold on the parity of Frobenius number. For the case of symmetric semigroups with three generators of Frobenius numbers we found an exact formula, which in a sense is the sum of two Sylvester's formulaes. We prove that the fraction of the symmetric semigroups is vanishing in the weak limit. 
Vladimir L. Shchur