Communications on Applied Nonlinear Analysis ISSN: 1074-133X Vol 32 No. 10s (2025) 561 https://internationalpubls.com βˆͺ A Study on the Differential Value of Total Graph D. Muralidharan,1 M.S. Paulraj2 and D. Yokesh3 1Department of Mathematics, Sri Sairam Institute of Technology, Chennai, Tamil Nadu, India. 2Department of Mathematics, A.M. Jain College, Chennai, Tamil Nadu, India. 3Department of Mathematics, Anand Institute of Technology, Chennai, Tamil Nadu, India. E-Mail: murali.maths@sairamit.edu.in Article History: Received: 12-01-2025 Revised: 15-02-2025 Accepted: 01-03-2025 Abstract: Let 𝐺 = (𝑉, 𝐸) be a graph and X be a subset of V. Let 𝐡(𝑋) be the set of vertices in V βˆ’ X that has a neighbour in a set X. The differential of a set X, is defined as βˆ‚(X) which is |B(X)| βˆ’ |X| and the differential of a graph is βˆ‚(G) = max {βˆ‚(X)/X βŠ‚ V}. The total graph T (G) of a g raph G is the graph whose vertex set is V (G) βˆͺ E(G) with two vertices of T (G) being adjacent if and only if the corresponding elements of G are either adjacent or incident. In this paper, we study the differential value of total graph for some standard graphs and its bounds. Keywords: total graph, domination number Mathematics Subject Classification 05C38, 05C69 1. Introduction Throughout this paper, 𝐺 = (𝑉, 𝐸) is a simple finite graph of 𝑛 vertices. For theoretical terminology about graph which is not given here, we refer to Harary [7]. For a vertex v ∈ V, the open neighbourhood of 𝑣 is 𝑁 (𝑣) = {𝑒 ∈ 𝑉/𝑒𝑣 ∈ 𝐸} and the closed neighbourhood of the set 𝑁 [𝑣] = 𝑁 (𝑣) βˆͺ {𝑣}. For a set X βŠ‚ V, its open neighbourhood 𝑁(𝑋) = ⋃ 𝑁(𝑣)π‘£πœ–π‘‰ and 𝑁[𝑋] = 𝑁(𝑋) βˆͺ 𝑋 is the closed neighbourhood. A set 𝐷 βŠ† 𝑉 is a dominating set [6,10] of 𝐺 if every vertex in 𝑉 βˆ’ 𝐷 is adjacent to some vertex in 𝐷. The boundary 𝐡(𝑋) of a set 𝑋 is defined to be the set of vertices in 𝑉 βˆ’ 𝑋 dominated by vertices in 𝑋, that is 𝐡(𝑋) = (𝑉 βˆ’ 𝑋) ∩ 𝑁(𝑋). The differential πœ•(𝑋) of 𝑋 is defined as |𝐡(𝑋)| βˆ’ |𝑋|. The differential of a graph G is πœ•(𝐺) = π‘šπ‘Žπ‘₯{πœ•(𝑋)/𝑋 βŠ‚ 𝑉}. If 𝑆 βŠ‚ 𝑉 and πœ•(𝐺) = πœ•(𝑆), then S is a βˆ‚-set. The differential of a set was first defined by Hedetniemi and later studied by Mashburn et al. and Goddard and Henning [3,4,9,13]. A graph 𝐺 is complete graph if every distinct pair of vertices are adjacent. A complete bipartite graph is a special type of bipartite graph where every vertex of one set is connected to every other vertex of other set. A complete binary tree is a special type of binary tree where all the levels of the tree are filled completely except the lowest level nodes which are filled from as left as possible. A graph 𝐺 is said to be dominant differential if it contains a πœ• βˆ’ set which is also a dominating set. Some examples are complete graph and wheel graph. mailto:murali.maths@sairamit.edu.in Communications on Applied Nonlinear Analysis ISSN: 1074-133X Vol 32 No. 10s (2025) 562 https://internationalpubls.com 2 The total graph 𝑇 (𝐺) [1,2,5,8,11,12,14] of 𝐺 is the graph whose vertex set is 𝑉 (𝐺) βˆͺ 𝐸(𝐺) with two vertices of 𝑇 (𝐺) being adjacent if and only if the corresponding elements of 𝐺 are either adjacent or incident and |𝑉 (𝑇 (𝐺))| = 𝑙. In this paper, we study the differential value of total graph of some standard graphs and its bounds. 2. Results Theorem 1. For any graph G of n vertices, 1 ≀ βˆ‚(T(G)) ≀ n(nβˆ’1) 2 Theorem 2. If T (G) is the total graph of a graph G, then βˆ‚(T(G)) ≀ n(nβˆ’1) 2 for n β‰₯ 2. Proof. We have to prove by induction method. We have to prove that the result is true for n = 2. When n = 2, then πœ•(𝑇 (𝐺)) ≀ 2(2βˆ’1) 2 = 1. It is always true. We assume that the result is true for n = k. Then, πœ•(𝑇 (𝐺)) ≀ π‘˜(π‘˜βˆ’1) 2 . We have to prove that the result is true for 𝑛 = π‘˜ + 1. Consider 𝐺 with 𝑛 = π‘˜ + 1 vertices. Removing a vertex v ∈ T (G) and |V (T (G)) βˆ’ {v}| = k, hence by induction hypothesis, πœ•(𝑇(𝐺) βˆ’ {𝑣}) + k ≀ k(kβˆ’1) 2 + k = k2βˆ’k+2k 2 = k2+k 2 = k(k+1) 2 . So, πœ•(𝑇 (𝐺)) ≀ π‘˜(π‘˜+1) 2 . Therefore, the result is true for 𝑛 = π‘˜ + 1 and the result is true for all 𝑛. So, βˆ‚(T(G)) ≀ n(nβˆ’1) 2 . Theorem 3. If G is a complete graph, then βˆ‚(T(G)) = 𝑛(π‘›βˆ’1) 2 , for n β‰₯ 2. Proof. Given that 𝐺 is a complete graph. Consider 𝑆 is a πœ•βˆ’ set of 𝑇(𝐺). When 𝑛 is even, choose any arbitrary vertex 𝑒1in 𝑆 and choose next vertex 𝑒2 which is not adjacent with 𝑒1. Choose next vertex 𝑒3 which is not adjacent with 𝑒1 and 𝑒2. Continuing this process until |S| = n 2 . Clearly, 𝑆 is a dominant differential of 𝑇 (𝐺). Therefore, βˆ‚(T(G)) = |B(S)| βˆ’ |S| = [ n(n+1) 2 βˆ’ n 2 ] βˆ’ n 2 = n(nβˆ’1) 2 . If 𝑛 is odd, we choose the vertices in S as we discussed in the above case and S dominates all the vertices except one in the case. Therefore, βˆ‚(T(G)) = [ n(n+1) 2 βˆ’ nβˆ’1 2 βˆ’ 1] βˆ’ nβˆ’1 2 = n(nβˆ’1) 2 . In both the cases, βˆ‚(T(G)) = 𝑛(π‘›βˆ’1) 2 . Hence the proof. Theorem 4. Given a positive integer k, there exist a graph on n vertices whose total graph on 𝑙 vertices with πœ•(𝑇(𝐺)) = π‘˜ Proof. If 𝑛 is odd, we consider the graph with 𝑛 = π‘˜+5 2 vertices whose total graph has π‘˜+3 2 copies of π‘˜3 with exactly one vertex as common, say 𝑣. Clearly, πœ•βˆ’ set of 𝑇(𝐺) contains 𝑣 only. Hence πœ•(𝑇(𝐺)) = π‘˜. If 𝑛 is even, when 𝑛 = 4, we consider the circulant graph C4,2. For other cases, we consider the circulant graph G = C4,2 in which π‘˜βˆ’6 2 copies of P2 which is attached with a vertex, say u of C4,2. Let S be the πœ•βˆ’ set of above graphs. Consider S = {u, v} where 𝑣 βˆ‰ 𝑁(𝑒). So, πœ•(𝑇(𝐺)) = π‘˜. Communications on Applied Nonlinear Analysis ISSN: 1074-133X Vol 32 No. 10s (2025) 563 https://internationalpubls.com Observation 5. For any graph G with n vertices, βˆ‚(T(G)) β‰  2. Theorem 6. For any graph G with n vertices, 1 ≀ βˆ‚(T(G)) βˆ’ βˆ‚(G) ≀ n2βˆ’3n+4 2 Proof. Given that 𝐺 is a graph of 𝑛 vertices. The maximum value of the differential of any total graph is less than or equal to 𝑛(π‘›βˆ’1) 2 and clearly πœ•(𝐺) ≀ 𝑛 βˆ’ 2. Therefore, βˆ‚(T(G)) βˆ’ βˆ‚(G) ≀ 𝑛(π‘›βˆ’1) 2 βˆ’ (𝑛 βˆ’ 2) = n2βˆ’3n+4 2 . Observation 7. For any graph G with n vertices, βˆ‚(T(G)) βˆ’ βˆ‚(G) = n2βˆ’3n+4 2 if and only if G is a complete graph. Theorem 8. If G = 𝐾1,π‘›βˆ’1 is a star graph, then βˆ‚ (T(𝐾1,π‘›βˆ’1)) = 2𝑛 βˆ’ 3 Proof. Let 𝑉(𝐾1,π‘›βˆ’1) = {𝑣, 𝑣1 , 𝑣2β€¦π‘£π‘›βˆ’1} and 𝐸(𝐾1,π‘›βˆ’1) = {𝑒1, 𝑒2β€¦π‘’π‘›βˆ’1} where 𝑒𝑖 = 𝑣𝑣𝑖, 𝑖 = 1,2, …𝑛 βˆ’ 1 and 𝑣 is a head vertex of the star graph. By the definition of total graph, 𝑉(𝐾1,π‘›βˆ’1) = {𝑣1, 𝑣2β€¦π‘£π‘›βˆ’1, 𝑒1, 𝑒2β€¦π‘’π‘›βˆ’1} and |𝑉(𝐾1,π‘›βˆ’1)| = 2𝑛 + 1. Since 𝑑𝑒𝑔(𝑣) = 2𝑛 and 𝑆 = {𝑣} is the differential set, then βˆ‚ (T(𝐾1,π‘›βˆ’1)) = 2𝑛 βˆ’ 3 Theorem 9. For any graph G = 𝐢𝑛, then πœ•(𝑇(𝑃𝑛)) = { 3 ⌊ 2𝑛 5 βŒ‹ , 𝑛 ≑ 0,1,3 (π‘šπ‘œπ‘‘ 5) 3 ⌊ 2𝑛 5 βŒ‹ + 1, 𝑛 ≑ 4 (π‘šπ‘œπ‘‘ 5) 3 ⌊ 2𝑛 5 βŒ‹ + 2, 𝑛 ≑ 2 (π‘šπ‘œπ‘‘ 5) Proof. let 𝑉(𝐢𝑛) = {𝑣1, 𝑣2, … . . , 𝑣𝑛} be the vertices of cycle of length 𝑛 (𝑛 β‰₯ 3) and 𝐸(𝐢𝑛) = {𝑒1, 𝑒2, … . . , 𝑒𝑛} be the edges of the corresponding vertices {𝑣1, 𝑣2, … . . , 𝑣𝑛}. Then, 𝑉(𝑇(𝐢𝑛)) = {𝑣1, 𝑣2, … . . , 𝑣𝑛, 𝑒1, 𝑒2, … . . , 𝑒𝑛} and 𝐸(𝑇(𝐺)) = {𝑣𝑖𝑣𝑖+1/1 ≀ 𝑖 ≀ 𝑛 βˆ’ 1} βˆͺ {𝑒𝑖𝑒𝑖+1/1 ≀ 𝑖 ≀ 𝑛 βˆ’ 1} βˆͺ {𝑣𝑖𝑒𝑖/1 ≀ 𝑖 ≀ 𝑛} βˆͺ {𝑒𝑖𝑣𝑖+1/1 ≀ 𝑖 ≀ 𝑛 βˆ’ 1} βˆͺ {𝑒𝑛𝑣1, 𝑒𝑛𝑒1, 𝑣𝑛𝑣1} and hence |𝑉(𝑇(𝐢𝑛))| = 2𝑛. Let 𝑆 be the differential set of 𝑇(𝐢𝑛). When 𝑛 ≑ 0 (π‘šπ‘œπ‘‘ 5). Consider 𝑆 = {𝑣1, 𝑒3, 𝑣6, … . . , π‘£π‘›βˆ’4, π‘’π‘›βˆ’2} be a πœ• βˆ’ set and hence. |𝑆| = 2𝑛 5 and |𝐡(𝑆)| = 8𝑛 5 . Then, πœ•(𝑇(𝐺)) = |𝐡(𝑆)| βˆ’ |𝑆| = 3 ⌊ 2𝑛 5 βŒ‹. When 𝑛 ≑ 1 (π‘šπ‘œπ‘‘ 5). Consider the differential set 𝑆 = {𝑣1, 𝑒3, 𝑣6, … . . , π‘’π‘›βˆ’3, π‘£π‘›βˆ’5} and 𝐢(𝑆) = {π‘’π‘›βˆ’1, π‘£π‘›βˆ’1}. Clearly, |𝑆| = ⌊ 2𝑛 5 βŒ‹ and |𝐡(𝑆)| = ⌊ 8𝑛 5 βŒ‹. So, πœ•(𝑇(𝐺)) = 3 ⌊ 2𝑛 5 βŒ‹. When 𝑛 ≑ 3 (π‘šπ‘œπ‘‘ 5). Consider the differential set 𝑆 = {𝑣1, 𝑒3, 𝑣6, … . . , π‘’π‘›βˆ’5, π‘£π‘›βˆ’2} and 𝐢(𝑆) = {π‘’π‘›βˆ’1}. Clearly, |𝑆| = ⌊ 2𝑛 5 βŒ‹ and |𝐡(𝑆)| = ⌊ 8𝑛 5 βŒ‹. So, πœ•(𝑇(𝐺)) = |𝐡(𝑆)| βˆ’ |𝑆| = 3 ⌊ 2𝑛 5 βŒ‹. When 𝑛 ≑ 4 (π‘šπ‘œπ‘‘ 5) Consider the set 𝑆 = 𝑆1 βˆͺ 𝑆2 where 𝑆1 = {𝑣1, 𝑒3, 𝑣6, … . . , π‘’π‘›βˆ’6, π‘£π‘›βˆ’3} and 𝑆2 = {π‘£π‘›βˆ’1}. Then πœ•(𝑆2) = 1. So, πœ•(𝑇(𝐺)) = 3 ⌊ 2𝑛 5 βŒ‹ + 1. When 𝑛 ≑ 2 (π‘šπ‘œπ‘‘ 5). Consider the set 𝑆 = 𝑆1 βˆͺ 𝑆2 where 𝑆1 = {𝑣1, 𝑒3, 𝑣6, … . . , π‘’π‘›βˆ’4, π‘£π‘›βˆ’6} and 𝑆2 = {π‘£π‘›βˆ’1} . Then πœ•(𝑆2) = 2. So, πœ•(𝑇(𝐺)) = 3 ⌊ 2𝑛 5 βŒ‹ + 2. Theorem 10. For any path 𝐺 = 𝑃𝑛(𝑛 β‰₯ 5), then Communications on Applied Nonlinear Analysis ISSN: 1074-133X Vol 32 No. 10s (2025) 564 https://internationalpubls.com πœ•(𝑇(𝑃𝑛)) = { 3 ⌊ 2𝑛 βˆ’ 1 5 βŒ‹ , 𝑛 ≑ 1,3,4 (π‘šπ‘œπ‘‘ 5) 3 ⌊ 2𝑛 βˆ’ 1 5 βŒ‹ + 1, 𝑛 ≑ 2(π‘šπ‘œπ‘‘ 5) 3 ⌊ 2𝑛 βˆ’ 1 5 βŒ‹ + 2, 𝑛 ≑ 0 (π‘šπ‘œπ‘‘ 5) Proof. let 𝑉(𝑃𝑛) = {𝑣1, 𝑣2, … . 𝑣𝑛} be the vertices of path of length 𝑛 (𝑛 β‰₯ 3) and 𝐸(𝑃𝑛) = {𝑒1, 𝑒2, … . . , π‘’π‘›βˆ’1}. Then, 𝑉(𝑇(𝑃𝑛)) = {𝑣1, 𝑣2, … . . , 𝑣𝑛, 𝑒1, 𝑒2, … . . , π‘’π‘›βˆ’1} and |𝑉(𝑇(𝑃𝑛))| = 2𝑛 βˆ’ 1. When 𝑛 ≑ 1 (π‘šπ‘œπ‘‘ 5). Consider the differential set of a total graph 𝑆 = {𝑣2, 𝑒4, … . . , π‘’π‘›βˆ’2, π‘£π‘›βˆ’4} and 𝐢(𝑆) = {𝑣𝑛}. Then, |𝐡(𝑆)| = ⌊ 8π‘›βˆ’4 5 βŒ‹ and |𝑆| = ⌊ 2π‘›βˆ’1 5 βŒ‹. So, πœ•(𝑇(𝐺)) = 3 ⌊ 2π‘›βˆ’1 5 βŒ‹. When 𝑛 ≑ 3 (π‘šπ‘œπ‘‘ 5), Consider the differential set of a total graph 𝑆 = {𝑣2, 𝑒4, … . . , π‘’π‘›βˆ’4, π‘£π‘›βˆ’1}. Then πœ•(𝑇(𝐺)) = 3 ⌊ 2π‘›βˆ’1 5 βŒ‹. When 𝑛 ≑ 4 (π‘šπ‘œπ‘‘ 5). Consider the differential set 𝑆 = {𝑣2, 𝑒4, … . . , π‘’π‘›βˆ’5, π‘£π‘›βˆ’2} and 𝐢(𝑆) = {𝑣𝑛, π‘’π‘›βˆ’1}. Then πœ•(𝑇(𝐺)) = 3 ⌊ 2π‘›βˆ’1 5 βŒ‹. When 𝑛 ≑ 2 (π‘šπ‘œπ‘‘ 5), there are three possible differential sets 𝑆 = 𝑆1 βˆͺ {π‘£π‘›βˆ’1} or 𝑆 = 𝑆1 βˆͺ {𝑣𝑛} or 𝑆 = 𝑆1 βˆͺ {π‘’π‘›βˆ’1} where 𝑆1 = {𝑣2, 𝑒4, … . , π‘’π‘›βˆ’3, π‘£π‘›βˆ’5}. Then πœ•(𝑇(𝐺)) = 3 ⌊ 2π‘›βˆ’1 5 βŒ‹ + 1. When 𝑛 ≑ 0 (π‘šπ‘œπ‘‘ 5), the possible differential set 𝑆 = 𝑆1 βˆͺ 𝑆2 where 𝑆1 = {𝑣2, 𝑒4, … . , π‘’π‘›βˆ’6, π‘£π‘›βˆ’3} and 𝑆2 = {π‘’π‘›βˆ’2}. Then, πœ•(𝑇(𝐺)) = 3 ⌊ 2π‘›βˆ’1 5 βŒ‹ + 2. Theorem 11. If 𝐺 = πΎπ‘š1×𝑛1 is a complete bipartite graph, then πœ• (𝑇(πΎπ‘š1×𝑛1 )) = (π‘š1 + 1)𝑛 βˆ’ π‘š1. Proof. Since 𝐺 = πΎπ‘š1×𝑛1 is a complete bipartite graph, the vertex set can be partitioned into two disjoint non empty sets 𝑉1(𝐺) = {𝑒1, 𝑒2β€¦π‘’π‘š1 } and 𝑉2(𝐺) = {𝑣1, 𝑣2, … 𝑣𝑛1}. Here, 𝑉 (𝑇(πΎπ‘š1×𝑛1 )) = {𝑒𝑖/1 ≀ 𝑖 ≀ π‘š1} βˆͺ {𝑣𝑗/1 ≀ 𝑗 ≀ 𝑛1} βˆͺ {𝑒𝑖𝑗/1 ≀ 𝑖 ≀ π‘š1, 1 ≀ 𝑗 ≀ 𝑛1} and |𝑉 (𝑇(πΎπ‘š1×𝑛1 ))| = π‘š1 + 𝑛1 +π‘š1𝑛1 . Clearly, the differential set 𝑆 = {𝑒1, 𝑒2β€¦π‘’π‘š1 }. Since 𝑒1 is adjacent with 2𝑛1 vertices and 𝑒2 is adjacent with 𝑛1 vertices and so on. Therefore πœ• (𝑇(πΎπ‘š1×𝑛1 )) = (2𝑛1 βˆ’ 1) + (𝑛1 βˆ’ 1) + β‹―+ (𝑛1 βˆ’ 1) = (π‘š1 + 1)𝑛1 βˆ’ π‘š1. Theorem 12. If 𝐺 is a complete binary tree, then πœ•(𝑇(𝐺)) = { 2π‘˜+3βˆ’7 3 , π‘˜ 𝑖𝑠 π‘Žπ‘› π‘œπ‘‘π‘‘ π‘–π‘›π‘‘π‘’π‘”π‘’π‘Ÿ 2π‘˜+3βˆ’5 3 , π‘˜ 𝑖𝑠 π‘Žπ‘› 𝑒𝑣𝑒𝑛 π‘–π‘›π‘‘π‘’π‘”π‘’π‘Ÿ Proof. Let Sk be the set of all vertices in level π‘˜ and |π‘†π‘˜| = 2 π‘˜ where 0 ≀ π‘˜ ≀ π‘š and π‘š is a positive integer. case(i) π‘˜ is an odd integer. Clearly, π‘†π‘˜βˆ’1 βˆͺ π‘†π‘˜βˆ’3 βˆͺ…βˆͺ 𝑆2 βˆͺ 𝑆0 is a πœ• βˆ’set. πœ•(𝑇(𝐺)) = 2π‘˜+1 + 2π‘˜βˆ’1 + 2π‘˜βˆ’3 +β‹―+ 24 + 22 βˆ’ 1 = 2π‘˜+1 [1 + 2π‘˜βˆ’1 2π‘˜+1 + 2π‘˜βˆ’3 2π‘˜+1 +β‹―+ 24 2π‘˜+1 + 22 2π‘˜+1 ] βˆ’ 1 Communications on Applied Nonlinear Analysis ISSN: 1074-133X Vol 32 No. 10s (2025) 565 https://internationalpubls.com = 2π‘˜+1[1 + 2βˆ’2 + 2βˆ’4 +β‹―+ 2βˆ’π‘˜+3 + 2βˆ’π‘˜+1] βˆ’ 1 = 2π‘˜+1 [1 + ( 1 22 ) 1 + ( 1 22 ) 2 + …+ ( 1 22 ) π‘˜βˆ’3 2 + ( 1 22 ) π‘˜βˆ’1 2 ] βˆ’ 1 = 2π‘˜+1 [ 1βˆ’( 1 22 ) π‘˜+1 2 1βˆ’( 1 22 ) ] βˆ’ 1 = 2π‘˜+1 [ 1βˆ’( 1 22 ) π‘˜+1 2 ( 3 22 ) ] βˆ’ 1 = 2π‘˜+1 Γ— 22 3 Γ— [ 1 βˆ’ 1 2π‘˜+1 ] βˆ’ 1 = [ 2π‘˜+3βˆ’4 3 ] βˆ’ 1 = [ 2π‘˜+3βˆ’7 3 ] case(ii) π‘˜ is an even integer. Clearly, π‘†π‘˜βˆ’1 βˆͺ π‘†π‘˜βˆ’3 βˆͺ…βˆͺ 𝑆3 βˆͺ 𝑆1 is a πœ• βˆ’set. πœ•(𝑇(𝐺)) = 2π‘˜+1 + 2π‘˜βˆ’1 + 2π‘˜βˆ’3 +β‹―+ 25 + 23 + 1 = 2π‘˜+1 [1 + 2π‘˜βˆ’1 2π‘˜+1 + 2π‘˜βˆ’3 2π‘˜+1 +β‹―+ 25 2π‘˜+1 + 23 2π‘˜+1 ] + 1 = 2π‘˜+1[1 + 2βˆ’2 + 2βˆ’4 +β‹―+ 2βˆ’π‘˜+4 + 2βˆ’π‘˜+2] + 1 = 2π‘˜+1 [1 + ( 1 22 ) 1 + ( 1 22 ) 2 + …+ ( 1 22 ) π‘˜βˆ’4 2 + ( 1 22 ) π‘˜βˆ’2 2 ] + 1 = 2π‘˜+1 [ 1βˆ’( 1 22 ) π‘˜ 2 1βˆ’( 1 22 ) ] + 1 = 2π‘˜+1 Γ— 22 3 Γ— [ 1 βˆ’ 1 2π‘˜ ] + 1 = [ 2π‘˜+3βˆ’8 3 ] + 1 = [ 2π‘˜+3βˆ’5 3 ] Theorem 13. If 𝐺 = 𝑃2 Γ— 𝑃𝑛1 is a grid graph with then πœ•(𝑇(𝑃2 Γ— 𝑃𝑛)) = 𝑙 βˆ’ 2𝑛1when 𝑛1 β‰₯ 2. Proof. Let 𝑇(𝑃2 Γ— 𝑃𝑛1) be the total graph of the grid graph and the vertex set of the graph is 𝑉 (𝑇(𝑃2 Γ— 𝑃𝑛1)) = {𝑣11, … 𝑣1𝑛1, 𝑣21, … 𝑣2𝑛1 , 𝑒11, . . , 𝑒1𝑛1 , 𝑒11, … 𝑒1(𝑛1βˆ’1), 𝑒21, … . 𝑒2(𝑛1βˆ’1)} where 𝑒𝑖𝑗 = 𝑣𝑖𝑗𝑣(𝑖+1)𝑗, 𝑖, 𝑗 = 1,2,… , 𝑛1 and |𝑉 (𝑇(𝑃2 Γ— 𝑃𝑛1))| = 5𝑛1 βˆ’ 2 = 𝑙. 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