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 A260912 Sum over the genera g of the number of immersions of an unoriented circle with n crossings in an unoriented surface of genus g. 4

%I #26 Sep 08 2022 08:46:13

%S 1,3,12,86,894,14715,313364,8139398,245237925,8382002270,319994166042,

%T 13492740284184,622738642693202,31225868370080949,1690360086869176780,

%U 98252177808632109236,6103194081506193327048,403488941845715112039425,28286698447226523233226110,2096044354918091666701275248

%N Sum over the genera g of the number of immersions of an unoriented circle with n crossings in an unoriented surface of genus g.

%C a(n) is the sum over the n-th row of triangle A260914.

%C a(n) is also the number of double cosets of H\G/K where G is the symmetric group S(2n), H is the subgroup generated by the centralizer of the circular permutation β = (1,2,3,...,2n) of G together with the permutation (2, 2n)(3, 2n-1)(4, 2n-2)...(n, n+2) that conjugates β and β-1, K is a subgroup of G generated by the permutation ρ = (1,2)(3,4)...(2n-3,2n-2)(2n-1,2n), using cycle notation, and the subgroup (isomorphic with S(n)) that commutes with ρ and permutes odd resp. even integers among themselves.

%C For g > 0 the immersions are understood up to stable geotopy equivalence (the counted curves cannot be immersed in a surface of smaller genus). - _Robert Coquereaux_, Nov 23 2015

%H R. Coquereaux, J.-B. Zuber, <a href="http://arxiv.org/abs/1507.03163">Maps, immersions and permutations</a>, arXiv preprint arXiv:1507.03163, 2015. Also J. Knot Theory Ramifications 25, 1650047 (2016), DOI: http://dx.doi.org/10.1142/S0218216516500474

%o (Magma) /* For all n */

%o nbofdblecos := function(G, H, K);

%o CG := Classes(G); nCG := #CG; oG := #G; CH := Classes(H); nCH := #CH; oH := #H; CK := Classes(K); nCK := #CK; oK := #K;

%o resH := []; for mu in [1..nCG] do Gmurep := CG[mu][3]; Hmupositions := {j: j in [1..nCH] | CycleStructure(CH[j][3]) eq CycleStructure(Gmurep)};

%o Hmugoodpositions := {j : j in Hmupositions | IsConjugate(G,CH[j][3], Gmurep) eq true}; bide := 0; for j in Hmugoodpositions do bide := bide + CH[j][2]; end for; Append(~resH, bide); end for;

%o resK := []; for mu in [1..nCG] do Gmurep := CG[mu][3]; Kmupositions := {j: j in [1..nCK] | CycleStructure(CK[j][3]) eq CycleStructure(Gmurep)};

%o Kmugoodpositions := {j : j in Kmupositions | IsConjugate(G,CK[j][3], Gmurep) eq true}; bide := 0; for j in Kmugoodpositions do bide := bide + CK[j][2]; end for; Append(~resK, bide); end for;

%o ndcl := 0; tot := 0; for mu in [1..nCG] do tot := tot + resH[mu]* resK[mu]/CG[mu][2]; end for; ndcl:= tot * oG/(oH * oK); return ndcl;

%o end function;

%o UUfull := function(n); G:=Sym(2*n); genH:={}; for j in [1..(n-1)] do v := G!(1,2*j+1)(2, 2*j+2); Include(~genH,v) ; end for;

%o H := PermutationGroup< 2*n |genH>;

%o beta:=G!Append([2..2*n],1); Cbeta:=Centralizer(G,beta); bool, rever := IsConjugate(G,beta,beta^(-1)); cycbeta := PermutationGroup< 2*n |{rever}>;

%o Cbetarev := sub<G|[Cbeta,cycbeta]>;

%o rho:=Identity(G); for j in [0..(n-1)] do v := G ! (2*j+1, 2*j+2) ; rho := rho*v ; end for; cycrho := PermutationGroup< 2*n |{rho}>;

%o Hcycrho:=sub<G|[H,cycrho]>;

%o return nbofdblecos(G,Hcycrho,Cbetarev); end function;

%o [UUfull(n) : n in [1..10]]; //

%Y Cf. A260847, A260887, A260296, A260914.

%K nonn

%O 1,2

%A _Robert Coquereaux_, Aug 04 2015

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Last modified September 10 06:17 EDT 2024. Contains 375773 sequences. (Running on oeis4.)