A028556 Palindromes of form k*(k+4).

0, 5, 77, 252, 525, 1221, 2112, 21312, 56165, 73437, 276672, 1265621, 2114112, 2509052, 29311392, 69555596, 71588517, 122434221, 126000621, 540888045, 5401221045, 21183638112, 69069096096, 71149694117, 79158385197, 2752360632572, 7320655560237, 12278611687221

Offset: 1

Author

Patrick De Geest

Keywords

• nonn
• base

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..49
• P. De Geest, Palindromic Quasipronics of the form n(n+x)

Crossrefs

Cf. A028347, A028555.

Programs

Mathematica

Select[Table[k(k+4),{k,0,3510000}],PalindromeQ] (* Harvey P. Dale, Apr 29 2025 *)

Python

from itertools import count, islice
def ispal(n): s = str(n); return s == s[::-1]
def agen():
    for k in count(0):
        if ispal(k*(k+4)):
            yield k*(k+4)
print(list(islice(agen(), 32))) # Michael S. Branicky, Jan 26 2022

Formula

a(n) = A028555(n) * (A028555(n) + 4). - Michael S. Branicky, Jan 26 2022

Extensions

a(26) and beyond from Michael S. Branicky, Jan 26 2022