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Sets, Relations, Lattices and Groups

GATE CSE 2025 Set 1 · Q49

MSQ2 marksMedium

Time

A={0,1,2,3,… }A = \{0, 1, 2, 3, \dots\} is the set of non-negative integers. Let F be the set of functions from AA to itself. For any two functions, f1,f2∈Ff_1, f_2 \in \text{F}, we define (f1⊙f2)(n)=f1(n)+f2(n)(f_1 \odot f_2)(n) = f_1(n) + f_2(n) for every number nn in AA. Which of the following is/are CORRECT about the mathematical structure (F,⊙)(\text{F}, \odot)?

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Solution

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Official answer: B) (F, ) is an Abelian monoid.

Sets, Relations, Lattices and Groups

Let A={0,1,2,3,… }A = \{0, 1, 2, 3, \dots\} and F={f∣f:A→A}F = \{f \mid f : A \to A\}. The binary operation ⊙\odot on FF is defined pointwise as (f1⊙f2)(n)=f1(n)+f2(n)(f_1 \odot f_2)(n) = f_1(n) + f_2(n) for all n∈An \in A.

Let us evaluate the algebraic properties of (F,⊙)(F, \odot):

  1. Closure: For any f1,f2∈Ff_1, f_2 \in F and n∈An \in A, both f1(n)∈Af_1(n) \in A and f2(n)∈Af_2(n) \in A. Since the sum of two non-negative integers is a non-negative integer, (f1⊙f2)(n)=f1(n)+f2(n)∈A(f_1 \odot f_2)(n) = f_1(n) + f_2(n) \in A. Hence, f1⊙f2∈Ff_1 \odot f_2 \in F.

  2. Associativity: For any f1,f2,f3∈Ff_1, f_2, f_3 \in F and n∈An \in A:

    ((f1⊙f2)⊙f3)(n)=(f1(n)+f2(n))+f3(n)=f1(n)+(f2(n)+f3(n))=(f1⊙(f2⊙f3))(n)((f_1 \odot f_2) \odot f_3)(n) = (f_1(n) + f_2(n)) + f_3(n) = f_1(n) + (f_2(n) + f_3(n)) = (f_1 \odot (f_2 \odot f_3))(n)

    Associativity holds because addition of integers is associative.

  3. Commutativity: For any f1,f2∈Ff_1, f_2 \in F and n∈An \in A:

    (f1⊙f2)(n)=f1(n)+f2(n)=f2(n)+f1(n)=(f2⊙f1)(n)(f_1 \odot f_2)(n) = f_1(n) + f_2(n) = f_2(n) + f_1(n) = (f_2 \odot f_1)(n)

    Commutativity holds because integer addition is commutative.

  4. Identity Element: Define e:A→Ae : A \to A by e(n)=0e(n) = 0 for all n∈An \in A. Since 0∈A0 \in A, e∈Fe \in F. For any f∈Ff \in F:

    (f⊙e)(n)=f(n)+e(n)=f(n)+0=f(n)(f \odot e)(n) = f(n) + e(n) = f(n) + 0 = f(n)

    Thus, ee serves as the identity element under ⊙\odot.

  5. Inverses: For an inverse g∈Fg \in F of a function f∈Ff \in F, we require (f⊙g)(n)=e(n)=0(f \odot g)(n) = e(n) = 0, meaning f(n)+g(n)=0f(n) + g(n) = 0 for all n∈An \in A. Since f(n),g(n)≥0f(n), g(n) \ge 0, this requires g(n)=−f(n)g(n) = -f(n). If f(n)>0f(n) > 0 for any nn, then g(n)<0∉Ag(n) < 0 \notin A, so g∉Fg \notin F. Thus, inverses do not exist for all elements in FF.

Since (F,⊙)(F, \odot) is associative, commutative, has an identity element, but lacks inverses for all elements, (F,⊙)(F, \odot) is an Abelian monoid.

Therefore, option B is correct.

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This Sets, Relations, Lattices and Groups previous year question is Q49 of the GATE CSE 2025 Set 1 paper. Find more GATE CSE PYQs on Sets, Relations, Lattices and Groups in Engineering Mathematics, or browse all GATE CSE PYQs.

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Sets, Relations, Lattices and Groups has 11 GATE CSE PYQs across 9 papers from 2014 to 2025. 3 of them are 2-mark MSQs like this one, and it is the only one in GATE CSE 2025 Set 1.

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GateAI solution cardGATE CSE 2025 Set 1 · Q49 · MSQ

A = {0, 1, 2, 3, } is the set of non-negative integers. Let F be the set of functions from A to itself. For any two functions, f1, f2 ∈ F, we define (f1 f2)(n) = f1(n) + f2(n) for every number n in A. Which of the following is/are CORRECT about the mathematical structure (F, )?

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Independent practice explanation verified by the GateAI team. GATE is conducted by the IITs; question text follows the official paper.