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Minimum Spanning Trees

GATE CSE 2012 · Q29

MCQ2 marksMedium

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Let GG be a weighted graph with edge weights greater than one and G′G^{\prime} be the graph constructed by squaring the weights of edges in GG. Let TT and T′T^{\prime} be the minimum spanning trees of GG and G′G^{\prime}, respectively, with total weights tt and t′t^{\prime}. Which of the following statements is TRUE?

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Minimum Spanning Trees
  1. Preservation of Relative Ordering: The function f(w)=w2f(w) = w^2 is strictly monotonically increasing for w>1w > 1. Kruskal's and Prim's algorithms depend only on the relative order of edge weights. Therefore, squaring the edge weights preserves the relative order of the weights, meaning T′=TT^{\prime} = T (assuming unique MSTs or corresponding choices of MSTs).

  2. Total Weight Comparison: Let the edge weights of the MST TT be w1,w2,…,wn−1w_1, w_2, \dots, w_{n-1}, where wi>1w_i > 1.

    • Total weight of TT: t=∑i=1n−1wit = \sum_{i=1}^{n-1} w_i

    • Total weight of T′T^{\prime}: t′=∑i=1n−1wi2t^{\prime} = \sum_{i=1}^{n-1} w_i^2

    • By algebraic expansion:

      t2=(∑i=1n−1wi)2=∑i=1n−1wi2+2∑1≤i<j≤n−1wiwj=t′+2∑i<jwiwjt^2 = \left(\sum_{i=1}^{n-1} w_i\right)^2 = \sum_{i=1}^{n-1} w_i^2 + 2 \sum_{1 \le i < j \le n-1} w_i w_j = t^{\prime} + 2 \sum_{i < j} w_i w_j
    • If the graph has n≥3n \ge 3 vertices, the cross terms are strictly positive, so t′<t2t^{\prime} < t^2.

    • If n=2n = 2, the tree has only 1 edge, so t′=t2t^{\prime} = t^2.

Because of edge cases (such as graphs with multiple MSTs or a 2-vertex graph where t′=t2t^{\prime} = t^2), the question was officially awarded Marks to All.

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This Minimum Spanning Trees previous year question is Q29 of the GATE CSE 2012 paper. Find more GATE CSE PYQs on Minimum Spanning Trees in Algorithms, or browse all GATE CSE PYQs.

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Minimum Spanning Trees in GATE CSE

Minimum Spanning Trees has 13 GATE CSE PYQs across 9 papers from 2011 to 2026. 5 of them are 2-mark MCQs like this one, and it is the only one in GATE CSE 2012.

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GateAI solution cardGATE CSE 2012 · Q29 · MCQ

Let G be a weighted graph with edge weights greater than one and G' be the graph constructed by squaring the weights of edges in G. Let T and T' be the minimum spanning trees of G and G', respectively, with total weights t and t'. Which of the following statements is TRUE?

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