IIT JAM MATH 2021 Question Paper | Sourav Sir's Classes

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JAM 2021 MATHEMATICS - MA
N = {1, 2, . . .}.
Z = {. . . , −2, −1, 0, 1, 2, . . .}.
Q = the set of rational numbers.
R = the set of real numbers.
Rn
= the n-dimensional real space with the Euclidean topology.
C = the set of complex numbers.
Cn
= the n-dimensional complex space with the Euclidean topology.
Mn(R), Mn(C) = the vector space of n × n real or complex matrices, respectively.
f0
, f00
= the first and second derivatives of the function f, respectively.
f(n)
= the nth. derivative of the function f.
R
C
stands for the line integral over the curve C.
In = the n × n identity matrix.
A−1
= the inverse of an invertible matrix A.
Sn = the permutation group on n symbols.
î = (1, 0, 0), ĵ = (0, 1, 0) and k̂ = (0, 0, 1).
ln x = the natural logarithm of x (to the base e).
|X| = the number of elements in a finite set X.
Zn = the additive group of integers modulo n.
arctan(x) denotes the unique θ ∈ (−π/2, π/2) such that tan θ = x.
All vector spaces are over the real or complex field, unless otherwise stated.
MA 1 / 1

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SECTION – A
MULTIPLE CHOICE QUESTIONS (MCQ)
Q. 1 – Q. 10 carry one mark each.
Q. 1 Let 0 < α < 1 be a real number. The number of differentiable functions y : [0, 1] → [0, ∞),
having continuous derivative on [0, 1] and satisfying
y0
(t) = (y(t))α
, t ∈ [0, 1],
y(0) = 0,
is
(A) exactly one. (B) exactly two.
(C) finite but more than two. (D) infinite.
Q. 2 Let P : R → R be a continuous function such that P(x) > 0 for all x ∈ R. Let y be a twice
differentiable function on R satisfying y00
(x) + P(x)y0
(x) − y(x) = 0 for all x ∈ R. Suppose
that there exist two real numbers a, b (a < b) such that y(a) = y(b) = 0. Then
(A) y(x) = 0 for all x ∈ [a, b]. (B) y(x) > 0 for all x ∈ (a, b).
(C) y(x) < 0 for all x ∈ (a, b). (D) y(x) changes sign on (a, b).
Q. 3 Let f : R → R be a continuous function satisfying f(x) = f(x + 1) for all x ∈ R. Then
(A) f is not necessarily bounded above.
(B) there exists a unique x0 ∈ R such that f(x0 + π) = f(x0).
(C) there is no x0 ∈ R such that f(x0 + π) = f(x0).
(D) there exist infinitely many x0 ∈ R such that f(x0 + π) = f(x0).
MA 1 / 17

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Q. 4 Let f : R → R be a continuous function such that for all x ∈ R,
Z 1
0
f(xt) dt = 0. (∗)
Then
(A) f must be identically 0 on the whole of R.
(B) there is an f satisfying (∗) that is identically 0 on (0, 1) but not identically 0 on the whole
of R.
(C) there is an f satisfying (∗) that takes both positive and negative values.
(D) there is an f satisfying (∗) that is 0 at infinitely many points, but is not identically zero.
Q. 5 Let p and t be positive real numbers. Let Dt be the closed disc of radius t centered at (0, 0),
i.e., Dt = {(x, y) ∈ R2
: x2
+ y2
≤ t2
}. Define
I(p, t) =
ZZ
Dt
dxdy
(p2 + x2 + y2)p
.
Then limt→∞ I(p, t) is finite
(A) only if p > 1. (B) only if p = 1.
(C) only if p < 1. (D) for no value of p.
Q. 6 How many elements of the group Z50 have order 10?
(A) 10 (B) 4 (C) 5 (D) 8
MA 2 / 17

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Q. 7 For every n ∈ N, let fn : R → R be a function. From the given choices, pick the statement
that is the negation of
“For every x ∈ R and for every real number 0, there exists an integer N 0 such that
Pp
i=1 |fN+i(x)| for every integer p 0.”
(A) For every x ∈ R and for every real number 0, there does not exist any integer N 0
such that
Pp
i=1 |fN+i(x)| for every integer p 0.
(B) For every x ∈ R and for every real number 0, there exists an integer N 0 such that
Pp
i=1 |fN+i(x)| ≥ for some integer p 0.
(C) There exists x ∈ R and there exists a real number 0 such that for every integer N 0,
there exists an integer p 0 for which the inequality
Pp
i=1 |fN+i(x)| ≥ holds.
(D) There exists x ∈ R and there exists a real number 0 such that for every integer N 0
and for every integer p 0 the inequality
Pp
i=1 |fN+i(x)| ≥ holds.
Q. 8 Which one of the following subsets of R has a non-empty interior?
(A) The set of all irrational numbers in R.
(B) The set {a ∈ R : sin(a) = 1}.
(C) The set {b ∈ R : x2
+ bx + 1 = 0 has distinct roots}.
(D) The set of all rational numbers in R.
Q. 9 For an integer k ≥ 0, let Pk denote the vector space of all real polynomials in one variable of
degree less than or equal to k. Define a linear transformation T : P2 −→ P3 by
Tf(x) = f00
(x) + xf(x).
Which one of the following polynomials is not in the range of T?
(A) x + x2
(B) x2
+ x3
+ 2 (C) x + x3
+ 2 (D) x + 1
MA 3 / 17

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Q. 10 Let n 1 be an integer. Consider the following two statements for an arbitrary n × n matrix
A with complex entries.
I. If Ak
= In for some integer k ≥ 1, then all the eigenvalues of A are kth
roots of unity.
II. If, for some integer k ≥ 1, all the eigenvalues of A are kth
roots of unity, then Ak
= In.
Then
(A) both I and II are TRUE. (B) I is TRUE but II is FALSE.
(C) I is FALSE but II is TRUE. (D) neither I nor II is TRUE.
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Q. 11 – Q. 30 carry two marks each.
Q. 11 Let Mn(R) be the real vector space of all n × n matrices with real entries, n ≥ 2.
Let A ∈ Mn(R). Consider the subspace W of Mn(R) spanned by {In, A, A2
, . . .}. Then the
dimension of W over R is necessarily
(A) ∞. (B) n2
. (C) n. (D) at most n.
Q. 12 Let y be the solution of
(1 + x)y00
(x) + y0
(x) −
1
1 + x
y(x) = 0, x ∈ (−1, ∞),
y(0) = 1, y0
(0) = 0.
Then
(A) y is bounded on (0, ∞). (B) y is bounded on (−1, 0].
(C) y(x) ≥ 2 on (−1, ∞). (D) y attains its minimum at x = 0.
Q. 13 Consider the surface S = {(x, y, xy) ∈ R3
: x2
+ y2
≤ 1}. Let ~
F = yî + xĵ + k̂. If n̂ is the
continuous unit normal field to the surface S with positive z-component, then
ZZ
S
~
F · n̂ dS
equals
(A) π
4
. (B) π
2
. (C) π. (D) 2π.
Q. 14 Consider the following statements.
I. The group (Q, +) has no proper subgroup of finite index.
II. The group (C {0}, ·) has no proper subgroup of finite index.
Which one of the following statements is true?
(A) Both I and II are TRUE. (B) I is TRUE but II is FALSE.
(C) II is TRUE but I is FALSE. (D) Neither I nor II is TRUE.
MA 5 / 17

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Q. 15 Let f : N → N be a bijective map such that
∞
X
n=1
f(n)
n2
+∞.
The number of such bijective maps is
(A) exactly one. (B) zero.
(C) finite but more than one. (D) infinite.
Q. 16 Define
S = lim
n→∞

1 −
1
22

1 −
1
32

· · ·

1 −
1
n2

.
Then
(A) S = 1/2. (B) S = 1/4. (C) S = 1. (D) S = 3/4.
Q. 17 Let f : R → R be an infinitely differentiable function such that for all a, b ∈ R with a b,
f(b) − f(a)
b − a
= f0
a + b
2

.
Then
(A) f must be a polynomial of degree less than or equal to 2.
(B) f must be a polynomial of degree greater than 2.
(C) f is not a polynomial.
(D) f must be a linear polynomial.
MA 6 / 17

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Q. 18 Consider the function
f(x) =
(
1 if x ∈ (R Q) ∪ {0},
1 − 1
p
if x = n
p
, n ∈ Z {0}, p ∈ N and gcd(n, p) = 1.
Then
(A) all x ∈ Q {0} are strict local minima for f.
(B) f is continuous at all x ∈ Q.
(C) f is not continuous at all x ∈ R Q.
(D) f is not continuous at x = 0.
Q. 19 Consider the family of curves x2
− y2
= ky with parameter k ∈ R. The equation of the
orthogonal trajectory to this family passing through (1, 1) is given by
(A) x3
+ 3xy2
= 4. (B) x2
+ 2xy = 3.
(C) y2
+ 2x2
y = 3. (D) x3
+ 2xy2
= 3.
Q. 20 Which one of the following statements is true?
(A) Exactly half of the elements in any even order subgroup of S5 must be even permutations.
(B) Any abelian subgroup of S5 is trivial.
(C) There exists a cyclic subgroup of S5 of order 6.
(D) There exists a normal subgroup of S5 of index 7.
Q. 21 Let f : [0, 1] → [0, ∞) be a continuous function such that
f(t)
2
1 + 2
Z t
0
f(s) ds, for all t ∈ [0, 1].
Then
(A) f(t) 1 + t for all t ∈ [0, 1]. (B) f(t) 1 + t for all t ∈ [0, 1].
(C) f(t) = 1 + t for all t ∈ [0, 1]. (D) f(t) 1 + t
2
for all t ∈ [0, 1].
MA 7 / 17

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Q. 22 Let A be an n×n invertible matrix and C be an n×n nilpotent matrix. If X =
X11 X12
X21 X22
!
is a 2n × 2n matrix (each Xij being n × n) that commutes with the 2n × 2n matrix B =
A 0
0 C
!
, then
(A) X11 and X22 are necessarily zero matrices.
(B) X12 and X21 are necessarily zero matrices.
(C) X11 and X21 are necessarily zero matrices.
(D) X12 and X22 are necessarily zero matrices.
Q. 23 Let D ⊆ R2
be defined by D = R2
{(x, 0) : x ∈ R}. Consider the function f : D → R
defined by
f(x, y) = x sin
1
y
.
Then
(A) f is a discontinuous function on D.
(B) f is a continuous function on D and cannot be extended continuously to any point outside
D.
(C) f is a continuous function on D and can be extended continuously to D ∪ {(0, 0)}.
(D) f is a continuous function on D and can be extended continuously to the whole of R2
.
Q. 24 Which one of the following statements is true?
(A) (Z, +) is isomorphic to (R, +).
(B) (Z, +) is isomorphic to (Q, +).
(C) (Q/Z, +) is isomorphic to (Q/2Z, +).
(D) (Q/Z, +) is isomorphic to (Q, +).
MA 8 / 17

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Q. 25 Let y be a twice differentiable function on R satisfying
y00
(x) = 2 + e−|x|
, x ∈ R,
y(0) = −1, y0
(0) = 0.
Then
(A) y = 0 has exactly one root.
(B) y = 0 has exactly two roots.
(C) y = 0 has more than two roots.
(D) there exists an x0 ∈ R such that y(x0) ≥ y(x) for all x ∈ R.
Q. 26 Let f : [0, 1] → [0, 1] be a non-constant continuous function such that f ◦ f = f. Define
Ef = {x ∈ [0, 1] : f(x) = x}.
Then
(A) Ef is neither open nor closed. (B) Ef is an interval.
(C) Ef is empty. (D) Ef need not be an interval.
Q. 27 Let g be an element of S7 such that g commutes with the element (2, 6, 4, 3). The number of
such g is
(A) 6. (B) 4. (C) 24. (D) 48.
Q. 28 Let G be a finite abelian group of odd order. Consider the following two statements:
I. The map f : G → G defined by f(g) = g2
is a group isomorphism.
II. The product
Q
g∈G g = e.
(A) Both I and II are TRUE. (B) I is TRUE but II is FALSE.
(C) II is TRUE but I is FALSE. (D) Neither I nor II is TRUE.
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Q. 29 Let n ≥ 2 be an integer. Let A : Cn
−→ Cn
be the linear transformation defined by
A(z1, z2, . . . , zn) = (zn, z1, z2, . . . , zn−1).
Which one of the following statements is true for every n ≥ 2?
(A) A is nilpotent. (B) All eigenvalues of A are of modulus 1.
(C) Every eigenvalue of A is either 0 or 1. (D) A is singular.
Q. 30 Consider the two series
I.
∞
X
n=1
1
n1+(1/n)
and II.
∞
X
n=1
1
n2−n1/n
.
Which one of the following holds?
(A) Both I and II converge. (B) Both I and II diverge.
(C) I converges and II diverges. (D) I diverges and II converges.
MA 10 / 17

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SECTION – B
MULTIPLE SELECT QUESTIONS (MSQ)
Q. 31 Let f : R → R be a function with the property that for every y ∈ R, the value of the expression
sup
x∈R
[xy − f(x)]
is finite. Define g(y) = supx∈R [xy − f(x)] for y ∈ R. Then
(A) g is even if f is even. (B) f must satisfy lim
|x|→∞
f(x)
|x|
= +∞.
(C) g is odd if f is even. (D) f must satisfy lim
|x|→∞
f(x)
|x|
= −∞.
Q. 32 Consider the equation
x2021
+ x2020
+ · · · + x − 1 = 0.
Then
(A) all real roots are positive. (B) exactly one real root is positive.
(C) exactly one real root is negative. (D) no real root is positive.
Q. 33 Let D = R2
{(0, 0)}. Consider the two functions u, v : D → R defined by
u(x, y) = x2
− y2
and v(x, y) = xy.
Consider the gradients ∇u and ∇v of the functions u and v, respectively. Then
(A) ∇u and ∇v are parallel at each point (x, y) of D.
(B) ∇u and ∇v are perpendicular at each point (x, y) of D.
(C) ∇u and ∇v do not exist at some points (x, y) of D.
(D) ∇u and ∇v at each point (x, y) of D span R2
.
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Q. 34 Consider the two functions f(x, y) = x + y and g(x, y) = xy − 16 defined on R2
. Then
(A) the function f has no global extreme value subject to the condition g = 0.
(B) the function f attains global extreme values at (4, 4) and (−4, −4) subject to the condition
g = 0.
(C) the function g has no global extreme value subject to the condition f = 0.
(D) the function g has a global extreme value at (0, 0) subject to the condition f = 0.
Q. 35 Let f : (a, b) → R be a differentiable function on (a, b). Which of the following statements
is/are true?
(A) f0
0 in (a, b) implies that f is increasing in (a, b).
(B) f is increasing in (a, b) implies that f0
0 in (a, b).
(C) If f0
(x0) 0 for some x0 ∈ (a, b), then there exists a δ 0 such that f(x) f(x0) for
all x ∈ (x0, x0 + δ).
(D) If f0
(x0) 0 for some x0 ∈ (a, b), then f is increasing in a neighbourhood of x0.
Q. 36 Let G be a finite group of order 28. Assume that G contains a subgroup of order 7. Which of
the following statements is/are true?
(A) G contains a unique subgroup of order 7.
(B) G contains a normal subgroup of order 7.
(C) G contains no normal subgroup of order 7.
(D) G contains at least two subgroups of order 7.
Q. 37 Which of the following subsets of R is/are connected?
(A) The set {x ∈ R : x is irrational}. (B) The set {x ∈ R : x3
− 1 ≥ 0}.
(C) The set {x ∈ R : x3
+ x + 1 ≥ 0}. (D) The set {x ∈ R : x3
− 2x + 1 ≥ 0}.
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Q. 38 Consider the four functions from R to R:
f1(x) = x4
+ 3x3
+ 7x + 1, f2(x) = x3
+ 3x2
+ 4x, f3(x) = arctan(x)
and
f4(x) =
(
x if x /
∈ Z,
0 if x ∈ Z.
Which of the following subsets of R are open?
(A) The range of f1. (B) The range of f2.
(C) The range of f3. (D) The range of f4.
Q. 39 Let V be a finite dimensional vector space and T : V → V be a linear transformation. Let
R(T) denote the range of T and N(T) denote the null space {v ∈ V : Tv = 0} of T. If
rank(T) = rank(T2
), then which of the following is/are necessarily true?
(A) N(T) = N(T2
). (B) R(T) = R(T2
).
(C) N(T)∩ R(T) = {0}. (D) N(T) = {0}.
Q. 40 Let m 1 and n 1 be integers. Let A be an m × n matrix such that for some m × 1 matrix
b1, the equation Ax = b1 has infinitely many solutions. Let b2 denote an m×1 matrix different
from b1. Then Ax = b2 has
(A) infinitely many solutions for some b2. (B) a unique solution for some b2.
(C) no solution for some b2. (D) finitely many solutions for some b2.
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SECTION – C
NUMERICAL ANSWER TYPE (NAT)
Q. 41 – Q. 50 carry one mark each.
Q. 41 The number of cycles of length 4 in S6 is .
Q. 42 The value of
lim
n→∞

3n
+ 5n
+ 7n
1
n
is .
Q. 43 Let B = {(x, y, z) ∈ R3
: x2
+ y2
+ z2
≤ 1} and define u(x, y, z) = sin (1 − x2
− y2
− z2
)2

for (x, y, z) ∈ B. Then the value of
ZZZ
B

∂2
u
∂x2
+
∂2
u
∂y2
+
∂2
u
∂z2

dxdydz
is .
Q. 44 Consider the subset S = {(x, y) : x2
+ y2
0} of R2
. Let
P(x, y) =
y
x2 + y2
and Q(x, y) = −
x
x2 + y2
for (x, y) ∈ S. If C denotes the unit circle traversed in the counter-clockwise direction, then
the value of
1
π
Z
C
(Pdx + Qdy)
is .
Q. 45 Consider the set A = {a ∈ R : x2
= a(a+1)(a+2) has a real root }. The number of connected
components of A is .
Q. 46 Let V be the real vector space of all continuous functions f : [0, 2] → R such that the restriction
of f to the interval [0, 1] is a polynomial of degree less than or equal to 2, the restriction of f
to the interval [1, 2] is a polynomial of degree less than or equal to 3 and f(0) = 0. Then the
dimension of V is equal to .
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Q. 47 The number of group homomorphisms from the group Z4 to the group S3 is .
Q. 48 Let y : 9
10
, 3

→ R be a differentiable function satisfying
(x − 2y)
dy
dx
+ (2x + y) = 0, x ∈

9
10
, 3

, and y(1) = 1.
Then y(2) equals .
Q. 49 Let ~
F = (y + 1)ey
cos(x)î + (y + 2)ey
sin(x)ĵ be a vector field in R2
and C be a continuously
differentiable path with the starting point (0, 1) and the end point (π
2
, 0). Then
Z
C
~
F · d~
r
equals .
Q. 50 The value of
π
2
lim
n→∞
cos
π
4

cos
π
8

· · · cos
π
2n+1

is .
MA 15 / 17

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Q. 51 The number of elements of order two in the group S4 is equal to .
Q. 52 The least possible value of k, accurate up to two decimal places, for which the following
problem
y00
(t) + 2y0
(t) + ky(t) = 0, t ∈ R,
y(0) = 0, y(1) = 0, y(1/2) = 1,
has a solution is .
Q. 53 Consider those continuous functions f : R → R that have the property that given any x ∈ R,
f(x) ∈ Q if and only if f(x + 1) ∈ R Q.
The number of such functions is .
Q. 54 The largest positive number a such that
Z 5
0
f(x)dx +
Z 3
0
f−1
(x)dx ≥ a
for every strictly increasing surjective continuous function f : [0, ∞) → [0, ∞) is .
Q. 55 Define the sequence
sn =

















1
2n
n−2
X
j=0
22j
if n 0 is even,
1
2n
n−1
X
j=0
22j
if n 0 is odd.
Define σm =
1
m
m
X
n=1
sn. The number of limit points of the sequence {σm} is .
MA 16 / 17

9
8
3
6
7
9
3
0
7
6
S
O
U
R
A
V
S
I
R
'
S
C
L
A
S
S
E
S
Q. 56 The determinant of the matrix





2021 2020 2020 2020
2021 2021 2020 2020
2021 2021 2021 2020
2021 2021 2021 2021





is .
Q. 57 The value of
lim
n→∞
Z 1
0
ex2
sin(nx) dx
is .
Q. 58 Let S be the surface defined by
{(x, y, z) ∈ R3
: z = 1 − x2
− y2
, z ≥ 0}.
Let ~
F = −yî + (x − 1)ĵ + z2
k̂ and n̂ be the continuous unit normal field to the surface S with
positive z-component. Then the value of
1
π
ZZ
S
∇ × ~
F

· n̂ dS
is .
Q. 59 Let A =



2 −1 3
2 −1 3
3 2 −1


. Then the largest eigenvalue of A is .
Q. 60 Let A =





1 0 0 0
0 1 0 0
0 0 −1 0
0 0 0 −1





. Consider the linear map TA from the real vector space M4(R)
to itself defined by TA(X) = AX − XA, for all X ∈ M4(R). The dimension of the range of
TA is .
END OF THE QUESTION PAPER
MA 17 / 17

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IIT JAM MATH 2021 Question Paper | Sourav Sir's Classes