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A371884 Irregular triangle read by rows in which row n >= 2 lists the divisors d of n such that d^n mod n = d.

%I A371884 #25 May 12 2025 14:37:45
%S A371884 1,1,1,1,1,3,1,1,1,1,5,1,1,4,1,1,7,1,5,1,1,1,9,1,1,5,1,7,1,11,1,1,1,1,
%T A371884 13,1,1,4,1,1,6,10,15,1,1,1,11,1,17,1,1,9,1,1,19,1,13,1,1,1,7,21,1,1,
%U A371884 1,9,1,23,1,1,16,1,1,25,1,17,1,13,1,1,27,1,11,1,8,1,19,1,29,1,1,1,1,31,1,1,1,5,13
%N A371884 Irregular triangle read by rows in which row n >= 2 lists the divisors d of n such that d^n mod n = d.
%H A371884 Robert Israel, <a href="/A371884/b371884.txt">Table of n, a(n) for n = 2..18962</a> (rows 2 to 10000, flattened)
%e A371884 Triangle begins:
%e A371884     1;
%e A371884     1;
%e A371884     1;
%e A371884     1;
%e A371884     1, 3;
%e A371884     1;
%e A371884     1;
%e A371884     1;
%e A371884     1, 5;
%e A371884     1;
%e A371884     1, 4;
%e A371884     1;
%e A371884     1, 7;
%e A371884     1, 5;
%e A371884     1;
%e A371884     1;
%e A371884     1, 9;
%e A371884     1;
%e A371884     1, 5;
%e A371884     1, 7;
%e A371884     1, 11;
%e A371884     1;
%e A371884     1;
%e A371884     1;
%e A371884     1, 13;
%e A371884     1;
%e A371884     1, 4;
%e A371884     1;
%e A371884     1, 6, 10, 15;
%e A371884     ...
%p A371884 f:= proc(n) op(sort(convert(select(d -> d^n mod n = d, numtheory:-divisors(n)),list))) end proc:
%p A371884 for n from 2 to 100 do f(n) od; # _Robert Israel_, May 11 2025
%t A371884 row[n_] := Select[Divisors[n], PowerMod[#, n, n] == # &]; Array[row, 64, 2] // Flatten (* _Amiram Eldar_, Apr 11 2024 *)
%o A371884 (Magma) [[d: d in Divisors(n) | d^n mod n eq d]: n in [2..65]];
%Y A371884 Cf. A027750, A182816, A279024, A371513, A371883.
%K A371884 nonn,tabf,look
%O A371884 2,6
%A A371884 _Juri-Stepan Gerasimov_, Apr 10 2024