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singular at x, y, z = 0. The authors obtained the existence and multiplicity of positive solutions
by means of Krasnoselskii’s fixed point theorem. As there do not exists 0 < L < M such that
(3.13) of [15] holds, the results of the multiple solutions are not correct in [15]. In [16], the
authors investigated the fractional differential equation
D
α
0+
u(t) + f(t, u(t), D
µ
0
+
u(t)) = 0, 0 < t < 1,
u(0) = u(1) = 0,
where α, µ ∈ R
1
+
and 1 < α ≤ 2, µ > 0 are real numbers, α − µ ≥ 1, D
α
0+
and D
µ
0
+
are
Riemann-Liouville fractional derivatives, f is a Carath´eodory function and f (t, x, y) is singular
at x = 0. The authors obtained the existence of positive solutions by means of Krasnoselskii’s
fixed point theorem. In [17], the authors investigated the fractional differential equation
D
α
0+
u(t) + f(t, u(t), u
′
(t), D
µ
0
+
u(t)) = 0, 0 < t < 1,
u(0) = 0, u
′
(0) = u
′
(1) = 0,
where α, µ ∈ R
1
+
and 2 < α ≤ 3, 0 < µ ≤ 1 are real numbers, D
α
0+
and D
µ
0
+
are Riemann-
Liouville fractional derivatives, f is a Carath´eodory function and f (t, x, y, z) is singular at
x, y, z = 0. The authors obtained the existence of positive solutions by means of Krasnoselskii’s
fixed point theorem. In [18], the authors investigated the singular problem
D
α
0+
u(t) + q(t)f (t, u(t), u
′
(t), · · · , u
(n−2)
(t)) = 0, 0 < t < 1,
u(0) = u
′
(0) = u
′′
(0) = · · · = u
(n−2)
(0) = u
(n−2)
(1) = 0,
where α ∈ R
1
+
and n − 1 < α ≤ n, n ≥ 2, the nonlinear function f(t, x
1
, x
2
, · · · , x
n−1
) may be
singular at x
i
= 0 (i = 1, 2, · · · , n − 1) and q(t) may be singular at t = 0. The existence results
of positive solutions are obtained by a fixed point theorem for the mixed monotone operator.
Motivated by the results above, in this paper, we utilize the sequential technique and
regularization to investigate the existence of positive solutions of BVP(1.1), where u ∈ C[0, 1]
is said to be a positive solution of BVP (1.1) if and only if u satisfies (1.1) and u(t) > 0
for any t ∈ (0, 1]. By using the sequential technique and regularization on a cone, some new
existence results are obtained for the case where the nonlinearity is allowed to be singular
with respect to both the time variable and the space variable. We should address here that
our work presented in this paper has various new features. Firstly, our study is on singular
nonlinear differential equation boundary value problems, that is, f(t, x
1
, x
2
, · · · , x
n−1
) may have
singularity at t = 0, 1 and x
i
= 0 (i = 1, 2, · · · , n−1), which leads to many difficulties in analysis.
Secondly, compared with [15−17], we complete the proof without the need of imposing the third
condition of the Carath´eodory conditions, that is, the condition |f(t, x
1
, x
2
, · · · , x
n−1
)| ≤ φ
H
(t)
is successfully removed, and at the same time the condition
lim
x→∞
h(x, x, · · · , x)
x
= 0
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