A028868 Primes that when squared give numbers with digits in nonascending order.

2, 3, 29, 31

Offset

1,1

Comments

No other solutions below 2*10^9 (probably finite). - Dec 15 1999

No other solutions below 10^32. - David A. Corneth, Oct 28 2023

Links

Table of n, a(n) for n=1..4.

David A. Corneth, PARI program

Example

From David A. Corneth, Oct 28 2023: (Start)
31 is in the sequence as 31 is prime and 31^2 = 961 which has its digits in nonascending order.
2979 is not in the sequence even though 2979^2 = 8874441 does have digits in nonascending order but 2979 = 3^2 * 331 is not prime. (End)

Prog

(PARI) is(n) = my(d = digits(n^2)); d == vecsort(d, , 4) && isprime(n) \\ David A. Corneth, Oct 28 2023
(PARI) \\ see link for a faster program

Crossrefs

Cf. A028869, A028865, A028866.

Sequence in context: A371024 A351693 A324941 * A081332 A106868 A098419

Adjacent sequences: A028865 A028866 A028867 * A028869 A028870 A028871

Keyword

nonn,base,hard,more

Author

Patrick De Geest

Extensions

Name clarified by Jon E. Schoenfield, Oct 27 2023

Status

approved