A390601 Positive integers (not multiples of 10) whose (radix-10) constant congruence speed is greater than the minimum of the (radix-10) constant congruence speeds of all their prime factors.

15, 18, 24, 25, 26, 32, 45, 51, 55, 57, 65, 68, 74, 75, 76, 82, 85, 93, 95, 99, 105, 115, 118, 124, 126, 132, 135, 143, 145, 155, 165, 168, 174, 175, 176, 182, 185, 195, 201, 205, 207, 215, 218, 224, 225, 226, 232, 235, 243, 249, 255, 265, 268, 274, 275, 276

Offset

1,1

Comments

Consider the radix-10 numeral system. For the definition of "constant congruence speed", see A373387 (see also the comments to A391434).

In general, if the product of two positive integers r and s is not divisible by 10, then the constant congruence speed of r*s is necessarily >= the minimum of the constant congruence speeds of r and s: the present sequence arises from the > case.

By definition, this sequence contains no primes (since the constant congruence speed of any prime equals that of its unique prime divisor).

Links

Table of n, a(n) for n=1..56.

Marco Ripà and Gabriele Di Pietro, A Compact Notation for Peculiar Properties Characterizing Integer Tetration, Zenodo, 2025.

Example

a(1) = 15 since the constant congruence speed of 3 is 1, the constant congruence speed of 5 is 2, the constant congruence speed of 15 is 4, and 4 > min{1,2}.

Crossrefs

Cf. A317905, A373387, A389432, A389979, A389980, A389981, A390320, A391434, A391695, A392231, A392233, A392565.

Subsequence of A067251.

Sequence in context: A379904 A290015 A354818 * A390320 A389981 A384947

Adjacent sequences: A390598 A390599 A390600 * A390602 A390603 A390604

Keyword

nonn,base

Author

Marco Ripà and Gabriele Di Pietro, Jan 20 2026

Status

approved