OFFSET
1,2
COMMENTS
All rep'n'-digits have infinite subsequence, except the rep'n'-digits 3 (mod 9) and 6 (mod 9).
For 'n' is 1, we have the Kaprekar numbers (A145875), the repdigit numbers.
If length is 1 (mod 9), repdigit 1 is part of the sequence, 1111111111*1111111111 = 1234567900987654321 => 123456790 + 987654321 = 1111111111.
If length is 2 (mod 9), repdigit 5 is part of the sequence, 55555555555*55555555555 = 3086419753024691358025 => 30864197530 + 24691358025 = 5555555555.
If length is 4 (mod 9), repdigit 7 is part of the sequence, 7777 * 7777 = 60481729 => 6048 + 1729 = 7777.
If length is 5 (mod 9), repdigit 2 is part of the sequence.
If length is 7 (mod 9), repdigit 4 is part of the sequence.
If length is 8 (mod 9), repdigit 8 is part of the sequence.
Repdigit 9 is part of this sequence in every length.
For 'n' is 2, we have numbers where two digits are repeated, like 52525252.
The rep2-digits which are divisible by 9 have the following infinite subsequences:
If length is 2 (mod 22), rep2-digit 54 is a part of this sequence, 545454545454545454545454 * 454545454545454545454545 = 247933884297520661157024297520661157024793388430 => 247933884297520661157024 + 297520661157024793388430 = 545454545454545454545454
If length is 4 (mod 22), rep2-digit 27 is a part of this sequence.
If length is 6 (mod 22), rep2-digit 18 is a part of this sequence.
If length is 8 (mod 22), rep2-digit 63 is a part of this sequence.
If length is 10 (mod 22), rep2-digit 90 is a part of this sequence.
If length is 14 (mod 22), rep2-digit 36 is a part of this sequence.
If length is 16 (mod 22), rep2-digit 81 is a part of this sequence.
If length is 18 (mod 22), rep2-digit 72 is a part of this sequence.
If length is 20 (mod 22), rep2-digit 45 is a part of this sequence.
Other rep2-digits also have infinite subsequences with length l (mod 198).
Example: rep2-digit 52 has length 8: 52525252 * 25252525 = 1326395239261300 => 13263952 + 39261300 = 52525252, the next length is 206.
LINKS
Pieter Post and Giovanni Resta, Table of n, a(n) for n = 1..187 (terms < 10^12, first 84 terms from Pieter Post)
EXAMPLE
124660 is a term. Indeed 124660*66421 = 8280041860 and 82800 + 41860 = 124660.
MATHEMATICA
fQ[n_] := Block[{c, d, len}, c = n FromDigits@ Reverse@ IntegerDigits@ n; d = IntegerDigits@ c; len = Length@ d; If[OddQ@ len, d = PadLeft[d, len + 1]; len++]; n == FromDigits@ Take[d, len/2] + FromDigits@ Take[d, -len/2]]; Select[Range@ 1000000, fQ] (* Michael De Vlieger, Jul 20 2015 *)
PROG
(Python)
def sod(n, m):
kk = 0
while n > 0:
kk= kk+(n%m)
n =int(n//m)
return kk
uu=1
for a in range (1, 9):
for n in range (10**(a-1)+1, 10**a):
y=int(str(n)[::-1])
ll=int(len(str(n*y))/2+0.5)
u=sod(n*y, 10**ll)
if n==u:
print (n)
(Python)
# for rep2-digit
for f in range (12, 98):
aa=1
for i in range(1, 200):
aa=10**(2*i)+aa
c=f*aa
cc=str(c*int(str(c)[::-1]))
l=int(len(cc)/2)
cc1, cc2=int(cc[0:l]), int(cc[l:2*l+1])
if c==cc1+cc2:
print (c)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Pieter Post, Jun 24 2015
EXTENSIONS
Missing a(21) from Giovanni Resta, Jul 19 2015
STATUS
approved