A393121 a(n) is the least k for which A024360(k) = n, or a(n) = -1 if such k does not exist.

4, 420, 4620, 13860, 106260, 367080, 1021020, 1741740, 1492260, 5290740, 38198160, 42222180, 29609580, 19399380, 23483460, 66186120

Offset

1,1

Comments

a(25) = 446185740, and a(n) for n >= 17 and n != 25 is larger, if it exists. - Amiram Eldar, Feb 07 2026

Links

Table of n, a(n) for n=1..16.

Prog

(PARI) A024360(n) = {my(f = factor(n), n2 = n^2, s, b, c, d, g); s = if(n == 1 || (n%4 == 2), 0, 2^(omega(f)-1)); for(k = 1, #f~, f[k, 2] *= 2);  fordiv(f, d, g = n2\d; if(d <= g && (d+g) % 2 == 0, c = (d+g)\2; b = g-c; if(n < b && gcd(b, c) == 1, s--))); s; }
list(len) = {my(v = vector(len), c = 0, n = 1, i); while(c < len, i = A024360(n); if(i > 0 && i <= len && v[i] == 0, c++; v[i] = n); n++); v; } \\ Amiram Eldar, Feb 07 2026

Crossrefs

Cf. A024360.

Sequence in context: A287965 A215647 A228242 * A036771 A201980 A080321

Adjacent sequences: A393118 A393119 A393120 * A393122 A393123 A393124

Keyword

nonn,more

Author

Lorenzo Sauras Altuzarra, Feb 02 2026

Extensions

a(6)-a(10) from Michel Marcus, Feb 02 2026

a(11)-a(16) from Amiram Eldar, Feb 07 2026

Status

approved