A245361 -id:A245361 - cp's OEIS frontend

A287389 Both k and its reverse are one less than a square.

0, 3, 8, 80, 99, 323, 360, 575, 840, 4224, 5775, 9999, 32760, 36480, 36863, 42024, 84680, 349280, 808200, 829920, 848240, 998000, 999999, 3055503, 3272480, 3426200, 3640463, 3644280, 3682560, 5597955, 8462280, 8803088, 30481440, 32855823, 80622440, 99999999

Offset: 1

Bruno Berselli, May 24 2017

Contains A028504. - Robert Israel, May 25 2017

Except for the first term, the first digit of each term is either 3, 4, 5, 8 or 9. - Chai Wah Wu, May 25 2017

			32760 is in the sequence because 32760 = 181^2-1 and its reverse 6723 = 82^2 - 1.

Chai Wah Wu, Table of n, a(n) for n = 1..300 (terms n = 1..164 from Robert Israel)

Cf. A124664: both k and its reverse are one more than a square.

Cf. A004086, A005563, A028504, A061457, A245361.

r:= n-> (s-> parse(cat(s[-i]$i=1..length(s))))(""||n):
select(x-> issqr(r(x)+1), [n^2-1$n=1..10000])[]; # Alois P. Heinz, May 24 2017

Select[Range[0, 10^6], Function[n, Times @@ Boole@ Map[IntegerQ@ Sqrt@ # &, {n + 1, FromDigits@ Reverse@ IntegerDigits@ n + 1}] == 1]] (* Michael De Vlieger, May 24 2017 *)

isok(n) = issquare(n+1) && issquare(fromdigits(Vecrev(digits(n)))+1); \\ Michel Marcus, May 24 2017

A245362 Semiprimes whose reversal + 1 is a square.

Original entry on oeis.org

51, 323, 341, 422, 591, 993, 998, 4227, 4265, 5129, 5534, 5921, 5937, 8049, 8657, 8801, 9953, 32133, 32282, 32471, 32597, 32817, 34091, 34379, 36611, 36863, 38937, 42011, 42243, 42605, 53211, 53673, 55745, 57167, 57903, 59543, 82151, 86354, 86781, 88217, 88433

Offset: 1

K. D. Bajpai, Jul 18 2014

Semiprimes in A245361.

Similar sequence for primes at A167217.

			341 is in the sequence because 341 = 11 * 31, which is semiprime, and reversal(341) + 1 = 143 + 1 = 144 = 12^2.
591 is in the sequence because 591 = 3 * 197, which is semiprime, and reversal(591) + 1 = 195 + 1 = 196 = 14^2.

K. D. Bajpai, Table of n, a(n) for n = 1..1054

Cf. A000290, A001358, A167217, A245361.

Select[Range[10^5], PrimeOmega[#] == 2 && IntegerQ[Sqrt[FromDigits[Reverse[IntegerDigits[#]]] + 1]] &]

revint(n) = eval(concat(Vecrev(Str(n))))
select(n->bigomega(n)==2 && issquare(revint(n)+1), vector(100000, n, n)) \\ Colin Barker, Jul 20 2014

cp's OEIS Frontend

A287389 Both k and its reverse are one less than a square.

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