cp's OEIS Frontend

This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

Showing 1-3 of 3 results.

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

Original entry on oeis.org

2, 3, 29, 31
Offset: 1

Views

Author

Patrick De Geest

Keywords

Comments

No other solutions below 2*10^9 (probably finite). - Dec 15 1999
No other solutions below 10^20. - David A. Corneth, Oct 28 2023, recomputed after a remark from Max Alekseyev, David A. Corneth, Aug 20 2024
Primes p such that p^2 is in A062826. - Max Alekseyev, Aug 18 2024

Examples

			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)

Links

Crossrefs

Cf. A028869, A028865, A028866, A062826.

Programs

  • 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

Extensions

Name clarified by Jon E. Schoenfield, Oct 27 2023

A028865 Primes that when squared gives numbers with digits in nondecreasing order.

Original entry on oeis.org

2, 3, 5, 7, 13, 17, 37, 67, 83, 107, 167, 337, 367, 383, 587, 1667, 166667, 333337, 333367, 333667, 336667, 666667, 33333667, 33666667, 66666667, 333333367, 666666667, 1666666667, 3336666667, 33333366667, 66666666667, 166666666667, 333333333367, 333333333667, 333336666667
Offset: 1

Views

Author

Patrick De Geest

Keywords

Crossrefs

Cf. A028866, A028868, A028869.

Extensions

Title changed to reflect terms by Sean A. Irvine, Feb 12 2020
a(26)-a(35) from Giovanni Resta, Feb 13 2020

A028869 Squares of primes with digits in nonascending order.

Original entry on oeis.org

4, 9, 841, 961
Offset: 1

Views

Author

Patrick De Geest

Keywords

Comments

No other solutions below 4 * 10^18 (probably finite). - Dec 15 1999

Examples

			961 = 31^2 is in the sequence since its digits (9, 6, 1) are in descending order.
1369 = 37^2 is not in the sequence, since its digits in descending order are: 9, 6, 3, 1.

Crossrefs

Cf. A028868, A028865, A028866.

Programs

  • Mathematica
    Select[Prime[Range[100]]^2, IntegerDigits[#] == Sort[IntegerDigits[#], Less] &] (* Alonso del Arte, Aug 12 2016 *)

Extensions

Offset changed by Altug Alkan, Sep 09 2016
Name clarified by Jon E. Schoenfield, Oct 27 2023
Showing 1-3 of 3 results.

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

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