{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,9,11]],"date-time":"2025-09-11T19:15:31Z","timestamp":1757618131953,"version":"3.44.0"},"reference-count":33,"publisher":"Springer Science and Business Media LLC","issue":"4","license":[{"start":{"date-parts":[[2025,6,2]],"date-time":"2025-06-02T00:00:00Z","timestamp":1748822400000},"content-version":"tdm","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0"},{"start":{"date-parts":[[2025,6,2]],"date-time":"2025-06-02T00:00:00Z","timestamp":1748822400000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0"}],"funder":[{"name":"South African National Research Foundation","award":["132588, 129265"],"award-info":[{"award-number":["132588, 129265"]}]},{"DOI":"10.13039\/501100006565","name":"University of Johannesburg","doi-asserted-by":"crossref","id":[{"id":"10.13039\/501100006565","id-type":"DOI","asserted-by":"crossref"}]}],"content-domain":{"domain":["link.springer.com"],"crossmark-restriction":false},"short-container-title":["Graphs and Combinatorics"],"published-print":{"date-parts":[[2025,8]]},"abstract":"<jats:title>Abstract<\/jats:title>\n          <jats:p>A set <jats:italic>S<\/jats:italic> of vertices in a graph <jats:italic>G<\/jats:italic> is a dominating set of <jats:italic>G<\/jats:italic> if every vertex not in <jats:italic>S<\/jats:italic> has a neighbor in <jats:italic>S<\/jats:italic>, where two vertices are neighbors if they are adjacent. If <jats:italic>G<\/jats:italic> is isolate-free, then a set <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$S \\subseteq V(G)$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>S<\/mml:mi>\n                    <mml:mo>\u2286<\/mml:mo>\n                    <mml:mi>V<\/mml:mi>\n                    <mml:mo>(<\/mml:mo>\n                    <mml:mi>G<\/mml:mi>\n                    <mml:mo>)<\/mml:mo>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> is a double dominating set of <jats:italic>G<\/jats:italic> if every vertex in <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$V(G) \\setminus S$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>V<\/mml:mi>\n                    <mml:mo>(<\/mml:mo>\n                    <mml:mi>G<\/mml:mi>\n                    <mml:mo>)<\/mml:mo>\n                    <mml:mo>\\<\/mml:mo>\n                    <mml:mi>S<\/mml:mi>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> has at least two neighbors in <jats:italic>S<\/jats:italic>, and every vertex in <jats:italic>S<\/jats:italic> has at least one neighbor in <jats:italic>S<\/jats:italic>. A double coalition in <jats:italic>G<\/jats:italic> consists of two disjoint sets of vertices <jats:italic>X<\/jats:italic> and <jats:italic>Y<\/jats:italic> of <jats:italic>G<\/jats:italic>, neither of which is a double dominating set but whose union <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$X \\cup Y$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>X<\/mml:mi>\n                    <mml:mo>\u222a<\/mml:mo>\n                    <mml:mi>Y<\/mml:mi>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> is a double dominating set of <jats:italic>G<\/jats:italic>. Such sets <jats:italic>X<\/jats:italic> and <jats:italic>Y<\/jats:italic> are said to form a double coalition. A double coalition partition in <jats:italic>G<\/jats:italic> is a vertex partition <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\Psi = \\{V_1,V_2,\\ldots ,V_k\\}$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>\u03a8<\/mml:mi>\n                    <mml:mo>=<\/mml:mo>\n                    <mml:mo>{<\/mml:mo>\n                    <mml:msub>\n                      <mml:mi>V<\/mml:mi>\n                      <mml:mn>1<\/mml:mn>\n                    <\/mml:msub>\n                    <mml:mo>,<\/mml:mo>\n                    <mml:msub>\n                      <mml:mi>V<\/mml:mi>\n                      <mml:mn>2<\/mml:mn>\n                    <\/mml:msub>\n                    <mml:mo>,<\/mml:mo>\n                    <mml:mo>\u2026<\/mml:mo>\n                    <mml:mo>,<\/mml:mo>\n                    <mml:msub>\n                      <mml:mi>V<\/mml:mi>\n                      <mml:mi>k<\/mml:mi>\n                    <\/mml:msub>\n                    <mml:mo>}<\/mml:mo>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> such that for all <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$i \\in [k]$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>i<\/mml:mi>\n                    <mml:mo>\u2208<\/mml:mo>\n                    <mml:mo>[<\/mml:mo>\n                    <mml:mi>k<\/mml:mi>\n                    <mml:mo>]<\/mml:mo>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula>, the set <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$V_i$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:msub>\n                    <mml:mi>V<\/mml:mi>\n                    <mml:mi>i<\/mml:mi>\n                  <\/mml:msub>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> forms a double coalition with another set <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$V_j$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:msub>\n                    <mml:mi>V<\/mml:mi>\n                    <mml:mi>j<\/mml:mi>\n                  <\/mml:msub>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> for some <jats:italic>j<\/jats:italic>, where <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$j \\in [k] \\setminus \\{i\\}$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>j<\/mml:mi>\n                    <mml:mo>\u2208<\/mml:mo>\n                    <mml:mo>[<\/mml:mo>\n                    <mml:mi>k<\/mml:mi>\n                    <mml:mo>]<\/mml:mo>\n                    <mml:mo>\\<\/mml:mo>\n                    <mml:mo>{<\/mml:mo>\n                    <mml:mi>i<\/mml:mi>\n                    <mml:mo>}<\/mml:mo>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula>. The double coalition number, <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\textrm{DC}(G)$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mtext>DC<\/mml:mtext>\n                    <mml:mo>(<\/mml:mo>\n                    <mml:mi>G<\/mml:mi>\n                    <mml:mo>)<\/mml:mo>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula>, of <jats:italic>G<\/jats:italic> equals the maximum order of a double coalition partition in <jats:italic>G<\/jats:italic>. We discuss the problem to determine or estimate the best possible constants <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\theta _{r}^{\\textrm{reg}}$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:msubsup>\n                    <mml:mi>\u03b8<\/mml:mi>\n                    <mml:mrow>\n                      <mml:mi>r<\/mml:mi>\n                    <\/mml:mrow>\n                    <mml:mtext>reg<\/mml:mtext>\n                  <\/mml:msubsup>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> and <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\theta _{r}$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:msub>\n                    <mml:mi>\u03b8<\/mml:mi>\n                    <mml:mi>r<\/mml:mi>\n                  <\/mml:msub>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> (which depend only on\u00a0<jats:italic>r<\/jats:italic>) for each <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$r \\ge 3$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>r<\/mml:mi>\n                    <mml:mo>\u2265<\/mml:mo>\n                    <mml:mn>3<\/mml:mn>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula>, such that <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\textrm{DC}(G) \\le \\theta _{r}^{\\textrm{reg}} \\times r$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mtext>DC<\/mml:mtext>\n                    <mml:mrow>\n                      <mml:mo>(<\/mml:mo>\n                      <mml:mi>G<\/mml:mi>\n                      <mml:mo>)<\/mml:mo>\n                    <\/mml:mrow>\n                    <mml:mo>\u2264<\/mml:mo>\n                    <mml:msubsup>\n                      <mml:mi>\u03b8<\/mml:mi>\n                      <mml:mrow>\n                        <mml:mi>r<\/mml:mi>\n                      <\/mml:mrow>\n                      <mml:mtext>reg<\/mml:mtext>\n                    <\/mml:msubsup>\n                    <mml:mo>\u00d7<\/mml:mo>\n                    <mml:mi>r<\/mml:mi>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> for the class of <jats:italic>r<\/jats:italic>-regular graphs <jats:italic>G<\/jats:italic> and <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\textrm{DC}(G) \\le \\theta _{r} \\times \\Delta (G)$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mtext>DC<\/mml:mtext>\n                    <mml:mrow>\n                      <mml:mo>(<\/mml:mo>\n                      <mml:mi>G<\/mml:mi>\n                      <mml:mo>)<\/mml:mo>\n                    <\/mml:mrow>\n                    <mml:mo>\u2264<\/mml:mo>\n                    <mml:msub>\n                      <mml:mi>\u03b8<\/mml:mi>\n                      <mml:mi>r<\/mml:mi>\n                    <\/mml:msub>\n                    <mml:mo>\u00d7<\/mml:mo>\n                    <mml:mi>\u0394<\/mml:mi>\n                    <mml:mrow>\n                      <mml:mo>(<\/mml:mo>\n                      <mml:mi>G<\/mml:mi>\n                      <mml:mo>)<\/mml:mo>\n                    <\/mml:mrow>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> for the class of graphs <jats:italic>G<\/jats:italic> with minimum degree equal to\u00a0<jats:italic>r<\/jats:italic>. We show that <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\theta _{r}^{\\textrm{reg}} \\ge 2 \\left( \\frac{r-1}{r} \\right) $$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:msubsup>\n                      <mml:mi>\u03b8<\/mml:mi>\n                      <mml:mrow>\n                        <mml:mi>r<\/mml:mi>\n                      <\/mml:mrow>\n                      <mml:mtext>reg<\/mml:mtext>\n                    <\/mml:msubsup>\n                    <mml:mo>\u2265<\/mml:mo>\n                    <mml:mn>2<\/mml:mn>\n                    <mml:mfenced>\n                      <mml:mfrac>\n                        <mml:mrow>\n                          <mml:mi>r<\/mml:mi>\n                          <mml:mo>-<\/mml:mo>\n                          <mml:mn>1<\/mml:mn>\n                        <\/mml:mrow>\n                        <mml:mi>r<\/mml:mi>\n                      <\/mml:mfrac>\n                    <\/mml:mfenced>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> for all <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$r \\ge 3$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>r<\/mml:mi>\n                    <mml:mo>\u2265<\/mml:mo>\n                    <mml:mn>3<\/mml:mn>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula>, and that equality holds if <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$r \\in \\{3,4\\}$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>r<\/mml:mi>\n                    <mml:mo>\u2208<\/mml:mo>\n                    <mml:mo>{<\/mml:mo>\n                    <mml:mn>3<\/mml:mn>\n                    <mml:mo>,<\/mml:mo>\n                    <mml:mn>4<\/mml:mn>\n                    <mml:mo>}<\/mml:mo>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula>, while <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\theta _{r} \\ge 2$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:msub>\n                      <mml:mi>\u03b8<\/mml:mi>\n                      <mml:mi>r<\/mml:mi>\n                    <\/mml:msub>\n                    <mml:mo>\u2265<\/mml:mo>\n                    <mml:mn>2<\/mml:mn>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> for all <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$r \\ge 3$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mi>r<\/mml:mi>\n                    <mml:mo>\u2265<\/mml:mo>\n                    <mml:mn>3<\/mml:mn>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula>, and <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\theta _{r} \\ge 3$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:msub>\n                      <mml:mi>\u03b8<\/mml:mi>\n                      <mml:mi>r<\/mml:mi>\n                    <\/mml:msub>\n                    <mml:mo>\u2265<\/mml:mo>\n                    <mml:mn>3<\/mml:mn>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> for <jats:italic>r<\/jats:italic> sufficiently large. Moreover, we show that <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$\\theta _3 = 2$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:msub>\n                      <mml:mi>\u03b8<\/mml:mi>\n                      <mml:mn>3<\/mml:mn>\n                    <\/mml:msub>\n                    <mml:mo>=<\/mml:mo>\n                    <mml:mn>2<\/mml:mn>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula>. Finally, we prove that <jats:inline-formula>\n              <jats:alternatives>\n                <jats:tex-math>$$5 \\le \\textrm{DC}(G) \\le 6$$<\/jats:tex-math>\n                <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\">\n                  <mml:mrow>\n                    <mml:mn>5<\/mml:mn>\n                    <mml:mo>\u2264<\/mml:mo>\n                    <mml:mtext>DC<\/mml:mtext>\n                    <mml:mo>(<\/mml:mo>\n                    <mml:mi>G<\/mml:mi>\n                    <mml:mo>)<\/mml:mo>\n                    <mml:mo>\u2264<\/mml:mo>\n                    <mml:mn>6<\/mml:mn>\n                  <\/mml:mrow>\n                <\/mml:math>\n              <\/jats:alternatives>\n            <\/jats:inline-formula> whenever <jats:italic>G<\/jats:italic> is a 4-regular graph.<\/jats:p>","DOI":"10.1007\/s00373-025-02937-2","type":"journal-article","created":{"date-parts":[[2025,6,2]],"date-time":"2025-06-02T14:45:30Z","timestamp":1748875530000},"update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Double Coalitions in Regular Graphs"],"prefix":"10.1007","volume":"41","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8185-067X","authenticated-orcid":false,"given":"Michael A.","family":"Henning","sequence":"first","affiliation":[]},{"given":"Doost Ali","family":"Mojdeh","sequence":"additional","affiliation":[]}],"member":"297","published-online":{"date-parts":[[2025,6,2]]},"reference":[{"issue":"11","key":"2937_CR1","doi-asserted-by":"publisher","first-page":"2283","DOI":"10.2989\/16073606.2024.2365365","volume":"47","author":"S Alikhani","year":"2024","unstructured":"Alikhani, S., Bakhshesh, D., Golmohammadi, H.: Total coalitions in graphs. 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