# On the saturated numerical semigroups

Sedat Ilhan 1  and Meral Süer 2
• 1 Department of Mathematics, Faculty of Sciences, Dicle University, Diyarbakır, Turkey
• 2 Faculty of Science and Literature, Batman University, Batman, Turkey
Sedat Ilhan
and Meral Süer

## Abstract

In this study, we characterize all families of saturated numerical semigroups with multiplicity four. We also present some results about invariants of these semigroups.

## 1 Introduction

Let ℕ = {1, 2, ..., n, ..} and ℤ be the set of integers. A subset S of the set ℕ of nonnegative integers is called a numerical semigroup if it satisfies the following conditions:

1. (i)0 ∈ S,
2. (ii)a, bSa + bS,
3. (iii)ℕ \ S has a finite number of elements.

Condition (iii) is equivalent to gcd(S) = 1 (Here, gcd(S) is the greatest common divisor of the element of S).

All numerical semigroups are finitely generated, i.e.
$S=〈a1,a2,...ar〉={∑k=1tciai,...,cr∈N}$
where a1, a2, ... ,arS and r ≥ 1. In this case, {a1, a2, ... , ar} is a minimal system of generators if no proper subset of {a1, a2, ... , ar} generates S. The numbers e(S) = r and m(S) = min{aS : a > 0} are called the embedding dimension and multiplicity of S respectively. In general, it holds that e(S) ≤ m(S). We say that S has maximal embedding dimension if e(S) = m(S) (see ).
We define the following invariants of numerical semigroups:
$F(S)=max{x:x∈Z∖S}$
and
$n(S)=|{0,1,2,...,F(S)}∩S|.$

F ( S ) and n ( S ) are called the Frobenius number of S and the number determiner of S, respectively.

We can write
$S=〈a1,a2,...,ar〉=〈s0=0,s1,s2,...,sn−1,sn=F(S)+1,→...〉$
where si < si+1 and n = n(S). The arrow means that every integer greater than F(S) + 1 belongs to S, for i = 1, 2, ... , n = n(S) (see ).
The set ℕ \ S is the gap of S, and the set of gaps of S is denoted by H(S). g(S) = |H(S)| is called the genus of S. It is clear that g(S) = F(S) + 1 − n(S). An element xH(S) is called a fundamental gap of S if 2x, 3xS. The set of all the fundamental gaps of S is denoted by FH(S), i.e.
$FH(S)={x∈H(S):2x,3x∈S}.$
An element x ∈ ℤ is called a Pseudo-Frobenius number of S if xS and x + sS, for sS \ {0}. We denote by PF(S) the set of all Pseudo-Frobenius numbers of S, i.e.
$PF(S)={x∈Z∖S:x+s∈S,foralls∈S∖{0}}$
(see ). Given a numerical semigroup S and xS \ {0}, we define the Apery set of x in S as Ap(S, x) = {sS : sxS} (for details see ).

If a numerical semigroup S satisfies the condition x + yzS, for every x, y, zS such that xyz, then S is called Arf. If S is an Arf numerical semigroup, then S has maximal embedding dimension.

The investigation of combinatorial properties of semigroups is very important, because they often occur in applications ([1, 3, 5]) and are related to automata theory (see ). A numerical semigroup S is saturated if s + c1s1 + c2s2 + ... + ckskS, where s, siS and ci ∈ ℤ such that c1s1 + c2s2 + ... + cksk ≥ 0 and sis for i = 1, 2, ... , k. Also, all saturated numerical semigroup are Arf. However an Arf numerical semigroup need not be to be saturated. The numerical semigroup
$S=〈7,12,15,16,17,18,20〉$
is Arf, but it is not saturated since 12 + (−5).7 + 3.12 = 13 ∉ S.

In this study, we show that all families of numerical semigroups with multiplicity four are saturated numerical semigroups; these are numerical semigroups of the form S = 〈4, k, k + 2, k + 3〉, for k = 3(mod4) and k ≥ 7 and S = 〈4, k, k + t, k + t + 2〉, for k = 2(mod 4) and k ≥ 6 and t an odd integer. We also give the formulae for F(S), n(S), PF(S), g(S), H(S) and FH(S) of these numerical semigroups.

## 2 Main results

In this section we provide some results for numerical semigroups with multiplicity four; i.e. numerical semigroups of the form S = 〈4, k, k + 2, k + 3〉 (for k ≡ 3(mod 4) and k ≥ 7) and S = 〈4, k, k + t, k + t + 2〉, (for k ≡ 2(mod 4) and k ≥ 6 and t an odd integer).

().Let S be a numerical semigroup, then the following conditions are equivalent:

1. (i)S is a saturated numerical semigroup.
2. (ii)a + dS(a)S for all aS, a > 0 where dS(a) = gcd{xS : xa }.
3. (iii)a + kdS(a)S for all aS, a > 0 and k ∈ ℕ.

(). If S = 〈4,k, k + 1, k + 2 〉 , then S is a saturated numerical semigroup, for k ≡ 1(mod 4) and k ≥ 5.

Let S = 〈4, k, k + 2, k + 3〉 be numerical semigroup, where k ≡ 3( mod 4) and k ≥ 7. Then S is saturated.

Let S = 〈4,k, k + 2, k + 3〉 be numerical semigroup, where k ≡ 3(mod 4) and k ≥ 7. We note that k = 4r + 3, r1 and r ∈ ℤ. Thus, we have
$S=〈4,k,k+2,k+3〉={0,4,8,⋯,k−7,k−3,k→⋯,}={0,4,8,⋯,4r−4,4r,4r+3,→⋯}.$

In this case,

1. (a)If a < 4r + 3, then dS(a) = 1. So, we find that a + dS(a)S since a + dS(a) = a + 14r + 4S, for all aS, a > 0.
2. (b)If a ≥ 4r + 3, then dS(a) = 4. So, we have a + dS(a) = a + 4 ∈ S, for all aS, a > 0.

In view of Proposition 2.1, we find that S is saturated a numerical semigroup. □

Let S = 〈4,k, k + t, k + t + 2〉 be numerical semigroup, where k ≡ 2(mod 4), k ≥ 6, and t is an odd integer. Then S is saturated.

It is trivial that gcd {4,k, k + t, k + t + 2} = 1 since k is even and t is an odd integer. If we put k = 4r + 2, r ≥ 1 and r ∈ ℤ, then we have
$S=〈4,k,k+t,k+t+2〉={0,4,8,...,k−6,k−2,k,k+2,...,k+t−3,k+t−1,→...,}={0,4,8,...,4r−4,4r,4r+2,4r+4,...,4r+t−1,4r+t+1,→...,}.$

In this case,

1. (i)If a > 4r + t + 1, then dS(a) = 1. So, we obtain a + dS(a)S from the inequality a + dS(a) = a + 1 ≥ 4r + t + 2 ∈ S, for all aS, a > 0.
2. (ii)If 4ra ≤ 4r + t + 1, then dS(a) = 2. So, we obtain a + dS(a) = a + 2 ∈ S from the inequality 4ra ≤ 4r + t + 1, for all aS, a > 0.
3. (iii)If a < 4r, then dS(a) = 4. So, we obtain a + dS(a) = a + 4 ∈ S since a + 4r < 4r + 4, for all aS, a > 0.

In view of Proposition 2.1, we have that S is a saturated numerical semigroup. □

().Let S be a numerical semigroup minimally generated by {n1 < n2 < ... < nr}. Then S has maximal embedding dimension if and only if Ap(S, n1) = {0,n2, n3,...,nr}.

().Let S be a numerical semigroup minimally generated by {n1 < n2 < ... < nr}. Then the following conditions are true:

1. (1)If S has maximal embedding dimension, then F(S) = nrn1.
2. (2)S has maximal embedding dimension if and only if
$g(S)=n2+n3+...+nrn1−n1−12.$

If S = 〈4, k k + 2, k + 3〉 is a numerical semigroup, where k ≡ 3(mod 4) and k ≡ 7. Then we obtain following equalities:

1. (a)F(S) = k − 1,
2. (b)$g(S)=3k−14$,
3. (c)PF(S) = {k − 4, k − 2, k − 1},
4. (d)$n(S)=k+14$,
5. (e)H(S) = {1, 2, 3, 5, 6, 7, ... , k − 6, k − 5, 5k − 4, k − 2, k − 1}.

We have Ap(S, 4) = {0, k, k − 2, k − 3} since S has maximal embedding dimension. Thus,

1. (a)We have F(S) = (k + 3) − 4 = k − 1 from Corollary 2.6 (1).
2. (b)We obtain $g(S)=k+k+2+k+34−4−12=3k−14$ from Corollary 2.6 (2).
3. (c)It is obvious that PF(S) = {k − 4, k + 2 − 4, k + 3 − 4}. So we find PF(S) = { k − 4, k − 2, k − 1}.
4. (d)We have $n(S)=(k−1)+1−3k−14=k+14$ from g(S) = F(S) + 1 − n(S).
5. (e)We find that H(S) = {1, 2, 3, 5, 6, 7, ... , 4r − 3, 4r − 2, 4r − 1, 4r + 1, 4r + 2} = {1, 2, 3, 5, 6, 7, ... , k − 6, k − 5, k − 4, k − 2, k − 1} from the equality S = < 4, k, k + 2, k + 3 > {0, 4, 8, ... , k − 7, k − 3, k, → ... ,} = {0, 4, 8, ... , 4r − 4, 4r, 4r − 3, → ... ,}.

Let S= 〈4, k, k + t, k + t + 2be a numerical semigroup, where k ≡ 2(mod 4), k ≡ 6, and t is an odd integer. Then, we have following equalities:

1. (a)F(S) = k + t − 2,
2. (b)$g(S)=3k+2t−44$,
3. (c)PF(S) = {k − 4, k + t − 4, k + t − 2},
4. (d)$n(S)=k+2t4$,
5. (e)H(S) = {1, 2, 3, 5, 6, 7, ... , k − 5, k − 4, k − 3, k − 1, k − 1, k + 3, ... , k + t − 2}.

We have Ap(S, 4) = {0, k, k + t, k + t + 2} since S has maximal embedding dimension. Thus,

1. (a)We have F(S) = (k + t + 2) − 4 = k + t + 2 from Corollary 2.6 (1).
2. (b)We obtain $g(S)=k+k+t+k+t+24−4−12=3k+2t−44$ from Corollary 2.6 (2).
3. (c)It is obvious that PF(S) = {k − 4, k + t − 4, k + t − 2 − 4}. So, we find PF(S) = {k − 4, k + t − 4, k + t − 2}.
4. (d)We have $n(S)=(k+t−2)+1−3k+2t−44=k+2t4$ from g(S) = F(S) + 1 − n(S).
5. (e)We observe that H(S) = {1, 2, 3, 5, 6, 7, ... , 4r − 3, 4r − 2, 4r − 1, 4r − 1, 4r + 3, 4r + 5, ... , 4r + t} = {1, 2, 3, 5, 6, 7, ... , k − 5, k − 4, k − 3, k − 1, k + 1, k + 3, ... , k + t + 2} since S= 〈4, k, k + t, k + t + 2〉 = {0, 4, 8, ... , k − 6, k − 2, k, k + 2, ... , k + t − 3, k + t1, → ... ,} = {0, 4, 8, ... ,4r − 4, 4r, 4r + 2, 4r + 4, ... , 4rt − 1, 4r + t + 1, → ... ,}.

Consider the numerical semigroup S= 〈4, k, k + 2, k + 3〉. If we put k = 15 then we have that S = 〈4, k, k + 2, k + 3〉 = 〈4, 15, 17, 18〉 = {0, 4, 8, 12, 15, → ... ,} is saturated. Hence, we find that

1. (a)F(S) = k − 1 = 15 − 1 = 14,
2. (b)$g(S)=3k−14=45−14=11$,
3. (c)PF(S) = {k − 4, k − 2, k − 1} = {11, 13, 14},
4. (d)$n(S)=k+14=15+14=4$,
5. (e)H(S) = {1, 2, 3, 5, 6, 7, ... , k − 6, k − 5, k − 4, k − 2, k − 1} = {1, 2, 3, 5, 6, 7, 9, 10, 11, 13, 14} and Ap(S, 4) = {0, k, k + 2, k + 3} = {0, 15, 17, 18}.

Consider the numerical semigroup S= 〈4, k, k + t, k + t + 2〉. If we put k = 14 and t = 13 then we find that S= 〈4, k, k + t, k + t + 2〉 = 〈4, 14, 27, 29〉 = {0, 4, 8, 12, 14, 16, 18, 20, 22, 24, 26, → ... ,} is saturated. Thus, we observe that

1. (a)F(S) = k + t − 2 = 14 + 13 − 2 = 25,
2. (b)$g(S)=3k+2t−44=644=16$,
3. (c)PF(S) = {k − 4, k + t − 4, k + t − 2} = {10, 23, 25},
4. (d)$n(S)=k+2t4=404=10$,
5. (e)H(S) = {1, 2, 3, 5, 6, 7, ... , k − 5, k − 4, k + 3, k − 1, k − 1, k +3, ... , kt − 2} = {1, 2, 3, 5, 6, 7, 9, 10, 11, 13, 15, 17, 19, 21, 23, 25} and Ap(S, 4) = {0, k, k + t, k + t + 2} = {0, 14, 27, 29}.

Acknowledgement

The authors thank the anonymous referee for his/her remarks which helped them to improve the presentation of the paper.

## References

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Abawajy J., Kelarev A.V., Chowdhury M., Power graphs: a survey, Electronic J. Graph Theory and Applications, 2013, 1(2), 125-147

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Barucci V, Dobbs D.E., Fontana M., Maximality properties in numerical semigroups and applications to one-dimensional analytically irreducible local domains,Memoirs of American Mathematical Society 1997, 125(598), 1-77

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Kelarev A.V., Combinatorial properties of sequences in groups and semigroups, "Combinatorics, Complexity and Logic", Discrete Mathematics and Theoretical Computer Science, 1996, 289-298

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Kelarev A.V., Graph Algebras and Automata, Marcel Dekker, New York, 2003

• 

Kelarev A.V., Ryan J., Yearwood J., Cayley graphs as classifiers for data mining:The influence of asymmetries, Discrete Mathematics, 2009, 309(17), 5360-5369

• 

Rosales J.C., Garcia-Sánchez P.A., Numerical Semigroups, Springer, 2009

• 

Rosales J.C., Garcia-Sánchez P.A., Garcia-Garcia J.I., Jimenez Madrid J.A., Fundamental gaps in numerical semigroups, Journal Pure and Applied Algebra, 2004, 189, 301-313

• 

Rosales J.C., Garcia-Sánchez P.A., Garcia-Garcia J.I., Branco M.B., Saturated numerical semigroups, Houston J.of Math. 2004, 30, 321-330

• 

Rosales J.C., Numerical Semigroups with Apery sets of unique expression, Journal of Algebra, 2000, 226, 479-487

• 

Suer M., Ilhan S., On a family of saturated numerical semigroup with multiplicity four, Turkish Journal of Math., (in press), DOI:10.3906

If the inline PDF is not rendering correctly, you can download the PDF file here.

• 

Abawajy J., Kelarev A.V., Chowdhury M., Power graphs: a survey, Electronic J. Graph Theory and Applications, 2013, 1(2), 125-147

• 

Barucci V, Dobbs D.E., Fontana M., Maximality properties in numerical semigroups and applications to one-dimensional analytically irreducible local domains,Memoirs of American Mathematical Society 1997, 125(598), 1-77

• 

Kelarev A.V., Combinatorial properties of sequences in groups and semigroups, "Combinatorics, Complexity and Logic", Discrete Mathematics and Theoretical Computer Science, 1996, 289-298

• 

Kelarev A.V., Graph Algebras and Automata, Marcel Dekker, New York, 2003

• 

Kelarev A.V., Ryan J., Yearwood J., Cayley graphs as classifiers for data mining:The influence of asymmetries, Discrete Mathematics, 2009, 309(17), 5360-5369

• 

Rosales J.C., Garcia-Sánchez P.A., Numerical Semigroups, Springer, 2009

• 

Rosales J.C., Garcia-Sánchez P.A., Garcia-Garcia J.I., Jimenez Madrid J.A., Fundamental gaps in numerical semigroups, Journal Pure and Applied Algebra, 2004, 189, 301-313

• 

Rosales J.C., Garcia-Sánchez P.A., Garcia-Garcia J.I., Branco M.B., Saturated numerical semigroups, Houston J.of Math. 2004, 30, 321-330

• 

Rosales J.C., Numerical Semigroups with Apery sets of unique expression, Journal of Algebra, 2000, 226, 479-487

• 

Suer M., Ilhan S., On a family of saturated numerical semigroup with multiplicity four, Turkish Journal of Math., (in press), DOI:10.3906

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