A361088 - OEIS

%I #19 Jun 11 2023 11:57:14

%S 1,2,2,2,3,3,2,2,4,2,4,2,4,2,4,3,4,3,3,4,2,4,2,6,2,6,2,4,6,2,4,2,4,4,

%T 5,4,4,5,4,5,5,2,2,6,2,6,6,2,6,2,6,4,4,2,6,4,4,2,8,4,4,2,8,4,2,8,2,8,

%U 3,8,3,3,4,4,6,2,4,4,6,2,8,4,6,2,8,2,6,2,8,2,6,2,8

%N Irregular table, read by rows, where row n holds the tau signature of n, i.e., the shortest sequence (tau(n+k), 0 <= k <= m) that uniquely identifies n; tau = A000005.

%C Row lengths are given by A309981(n) + 1; see there (and the OEIS wiki page) for examples.

%H M. F. Hasler et al, <a href="/wiki/A309981">tau signature</a>, OEIS Wiki, April 2023.

%e The first 20 rows read as follows:

%e    n | row n: tau-signature of n

%e   ---+--------------------------

%e    1 | [1]

%e    2 | [2, 2]

%e    3 | [2, 3]

%e    4 | [3, 2]

%e    5 | [2, 4, 2]

%e    6 | [4, 2, 4]

%e    7 | [2, 4, 3]

%e    8 | [4, 3]

%e    9 | [3, 4, 2]

%e   10 | [4, 2, 6]

%e   11 | [2, 6, 2, 4]

%e   12 | [6, 2, 4]

%e   13 | [2, 4, 4, 5]

%e   14 | [4, 4, 5]

%e   15 | [4, 5]

%e   16 | [5, 2]

%e   17 | [2, 6, 2, 6]

%e   18 | [6, 2, 6]

%e   19 | [2, 6, 4, 4, 2]

%e   20 | [6, 4, 4, 2, 8]

%e See the wiki page for proofs.

%o (PARI) signatures=Map(); LIMIT=10^5 /* This search limit should (possibly dynamically, or by hand) be increased as n grows beyond 100. As of today, the value for n=49 is not yet proven. */

%o A361088_row(n,s=0)={if(!s, s=iferr(mapget(signatures,n),E,[]); #s|| for(L=1,oo, s=concat(s,numdiv(n+L-1)); A361088_row(n,s)|| [mapput(signatures,n,[s,LIMIT]); return(s)]); s[2]>=LIMIT&& return(s[1]); s=s[1]; while(A361088_row(n,s), s=concat(s,numdiv(n+#r))); mapput(signatures,n,[s,LIMIT]); return(s)); my(r=iferr(mapget(signatures,s), E,[])); if(!r, r=[n,n], r[2]<n, r=[r[1],n,n]; mapput(signatures,s,r); return(n), #r>2, return(r[#r-1]), r[#r]>=LIMIT, return); for(j=max(r[2],n)+1,LIMIT, for(k=1,#s, numdiv(j+k-1)!=s[k]&& next(2)); mapput(signatures,s,[n,j,j]); return(j)); mapput(signatures,s,[n,LIMIT])

%Y Cf. A309981, A327265, A161460, A000005 (tau = numdiv).

%K nonn,tabf

%O 1,2

%A _M. F. Hasler_, Apr 07 2023