A342357 -id:A342357 - cp's OEIS frontend

1, 2, 6, 22, 342, 1444, 33184, 399235, 12502550, 117906198, 7740054144, 74673569118, 4724493959332, 121637216075836, 4503600768557056, 89450720590507768, 10119960926575526448, 152232968281237988010, 16384000020089600480000, 552020693349464673399080, 35271474934322858202723576

Offset: 1

Don Knuth, Feb 22 2021

nonn

Consider vertices numbered 0 to n. For 1 <= k <= n, add an arc from v_k to (v_k+k) mod q, where q = n+1. (Thus (n+1)^n possibilities.) Two such digraphs are considered equivalent under the following operations: (i) rename each v to (v+1) mod q; (ii) rename each v to (av) mod q, where a is relatively prime to q; (iii) form the converse digraph by reversing the direction of every arc. The number of equivalence classes is a(n).

			Each equivalence class has exactly one digraph with v_1=0.
For n=3 the six classes of digraphs 0v_2v_3 are: {000,032}, {001,011,021,031}, {002,010,022,030}, {003,033}, {012,020}, {013,023}.

G. S. Bloom and D. F. Hsu, On graceful digraphs and a problem in network addressing, Congr. Numer., 35 (1982) 91-103.
D. E. Knuth, The Art of Computer Programming, forthcoming exercise in Section 7.2.2.3

Peter Luschny, Table of n, a(n) for n = 1..100

Cf. A342357, A071118.

# This is a port from the Mathematica program below.
sols := proc(alf, bet, q)
igcd(alf, q); `if`(modp(bet, %) <> 0, 0, %) end:
F := proc(l, a, b, q) local s, r, ll;
   s := 1; r := 1; ll := modp(l*a, q);
   while ll > l do
      s := s + 1;
      ll := modp(ll*a,  q);
      r  := modp(r*a+1, q); od;
`if`(ll <> l, 1, sols(a^s - 1, -r*b, q)) end:
G := proc(l, a, b, q) local s, r, ll;
   s := 1; r := 1; ll := modp(-l*a, q);
   while ll > l do
      s := s + 1;
      ll := modp(-ll*a, q);
      r  := modp(r*a+1, q); od;
`if`(ll <> l, 1, sols(a^s - 1, -r*b - l*irem(s, 2), q)) end:
f := (a, b, q) -> mul(F(l, a, b, q), l = 1..q-1):
g := (a, b, q) -> mul(G(l, a, b, q), l = 1..q-1):
a := proc(n) local q; q := n + 1;
    add(`if`(igcd(a, q) > 1, 0, add(f(a, b, q) + g(a, b, q),
         b = 0..q-1)), a = 1..q-1);
% / (2*q*numtheory:-phi(q)) end:
seq(a(n), n=1..21); # Peter Luschny, Mar 10 2021

sols[alf_,bet_,q_]:=Block[{d=GCD[alf,q]},If[Mod[bet,d]!=0,0,d]]
(* that many solutions to alf x == bet (modulo q) for 0<=xl, s++;ll=Mod[ll*a,q];r=Mod[r*a+1,q]];
                       If[ll==l,sols[a^s-1,-r b,q],1]]
g[l_,a_,b_,q_]:=Block[{r,s,ll,atos},
                       s=1;ll=Mod[-l*a,q];r=1;
                       While[ll>l, s++;ll=Mod[-ll*a,q];r=Mod[r*a+1,q]];
                       If[ll==l,sols[a^s-1,-r b-l Mod[s,2],q],1]]
f[a_,b_,q_]:=Product[f[l,a,b,q],{l,q-1}]
g[a_,b_,q_]:=Product[g[l,a,b,q],{l,q-1}]
x[q_]:=Sum[If[GCD[a,q]>1,0,Sum[f[a,b,q]+g[a,b,q],{b,0,q-1}]],{a,q-1}]/
  (2q EulerPhi[q])
a[n_]:=x[n+1]

a(10) from Don Knuth, Mar 10 2021

More terms from Peter Luschny, Mar 10 2021

cp's OEIS Frontend

A341884 Number of fundamentally different graceful labelings of digraphs with n arcs.

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