1,2

a(n+1) >= a(n) for all n.

2^25 = 33554432 contains a 3-digit substring of "98765432109876543210987654..." (in this case, "432"). Since 25 is the lower power to have this property, a(3) = 25.

(Python)

def Rev(n):

rev = ''

for i in str(n):

rev = i + rev

return rev

def a(n):

lst = []

for b in range(1, 10**n):

if len(str(2**b)) >= n:

lst.append(b)

break

for k in range(lst[0], 50000):

for i in range(10):

s = ''

s += str(i)

for j in range(i+1, i+n):

dig = j%10

s+=str(dig)

if str(2**k).find(Rev(s)) > -1:

return k

n = 1

while n < 100:

print(a(n), end=', ')

n += 1

Cf. A243150.

Sequence in context: A301912 A171272 A366158 * A238449 A062989 A122679

Adjacent sequences: A243300 A243301 A243302 * A243304 A243305 A243306

nonn,base,hard,more

Derek Orr, Jun 04 2014

a(10)-a(13) from Hiroaki Yamanouchi, Sep 26 2014

a(14) from Chai Wah Wu, May 29 2020

approved