A124664 Both k and its reverse are one more than a square.

1, 2, 5, 10, 50, 101, 626, 730, 1090, 2210, 5477, 7745, 10001, 10610, 71290, 227530, 1000001, 1010026, 1014050, 1040401, 2217122, 2676497, 5053505, 5631130, 6200101, 6265010, 7946762, 100000001, 101808101, 248157010, 10000000001, 10180608202, 10182828101

Offset

1,2

Comments

The first digit for each term is either 1, 2, 5, 6 or 7. - Chai Wah Wu, May 25 2017

Links

Chai Wah Wu, Table of n, a(n) for n = 1..100

Erich Friedman, What's Special About This Number?. See 5477 and 7745.

Example

5477 is in the sequence because 5477 = 74^2 + 1 and 7745 = 88^2 + 1.

Maple

r:= n-> (s-> parse(cat(s[-i]$i=1..length(s))))(""||n):
select(x-> issqr(r(x)-1), [n^2+1$n=0..150000])[]; # Alois P. Heinz, May 24 2017

Mathematica

Select[Range[10000000], IntegerQ[Sqrt[ # - 1]] && IntegerQ[Sqrt[FromDigits[Reverse[IntegerDigits[ # ]]] - 1]] &]

Crossrefs

A066618 is a subsequence of this sequence of numbers that do not end in 0. The sequence A027720 = Palindromes of form n^2 + 1 - is a palindromic subsequence of this sequence.

Cf. A287389: both k and its reverse are one less than a square.

Sequence in context: A139010 A018472 A082593 * A076776 A020755 A018503

Adjacent sequences: A124661 A124662 A124663 * A124665 A124666 A124667

Keyword

nonn,base

Author

Tanya Khovanova, Dec 23 2006

Extensions

More terms from Alois P. Heinz, May 24 2017

Status

approved