OFFSET
1,3
COMMENTS
n-th row length gives 1 for n = 1, and 2^A001221(n) for n >= 2 , that is A034444(n). [Proof: Unique lifting theorem (e.g., Apostol, 5.30 (a), p.121) for this congruence, and only two solutions 1 and p for primes p. See also the Yuval Dekel, Sep 21 2003, comment in A034444. - Wolfdieter Lang, Feb 05 2020]
REFERENCES
Tom M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1986.
EXAMPLE
The irregular triangle T(n,k) begins
n\k 1 2 3 4 ...
1: 1
2: 1 2
3: 1 3
4: 1 4
5: 1 5
6: 1 3 4 6
7: 1 7
8: 1 8
9: 1 9
10: 1 5 6 10
11: 1 11
12: 1 4 9 12
13: 1 13
14: 1 7 8 14
15: 1 6 10 15
16: 1 16
17: 1 17
18: 1 9 10 18
19: 1 19
20: 1 5 16 20
...
MATHEMATICA
Table[Select[Range@ n, Mod[-n + # (# - 1), n] == 0 &], {n, 25}] // Flatten (* Michael De Vlieger, Nov 18 2019 *)
PROG
(Magma) [[k: k in [1..n] | k^2 mod n eq k]: n in [1..38]];
(PARI) row(n) = select(x->(Mod(x, n) == Mod(x, n)^2), [1..n]); \\ Michel Marcus, Nov 19 2019
CROSSREFS
KEYWORD
nonn,easy,tabf
AUTHOR
Juri-Stepan Gerasimov, Nov 15 2019
EXTENSIONS
Edited by Wolfdieter Lang, Feb 05 2020
STATUS
approved