“…Since 2+2+5 / ∈ S and 0+2+7 / ∈ S, we have K +K +5 S and K +K +7 S, while K +K +13 ⊆ S because 13 is greater than f (S). Hence the symmetric double of S with minimal genus is S ✶ 13 K = {0, 10,13,14,16,17,20,23,24,26,27,28,29,30,31,32,33,34,36 →} that has genus f (S) + b+1 2 = 18. Note that the minimum genus of a double of S is g(S) + ⌈ f (S) 2 ⌉ = 13 and, according to the proof of Theorem 2.1, it is obtained by the semigroup {0, 10,13,14,15,16,17,19,20,21,23 →}.…”