cp's OEIS Frontend

A081521 Triangle read by rows: row n contains n terms in increasing order, relatively prime to n, whose sum is a multiple of n and such that the row contains the smallest possible subset of consecutive numbers starting with 1.

%I A081521 #16 May 24 2020 04:02:29
%S A081521 1,1,3,1,4,7,1,3,5,7,1,2,3,6,8,1,5,7,11,13,17,1,2,3,4,5,8,12,1,3,5,7,
%T A081521 9,11,13,15,1,2,4,5,7,8,10,13,22,1,3,7,9,11,13,17,19,21,29,1,2,3,4,5,
%U A081521 6,7,8,9,12,20
%N A081521 Triangle read by rows: row n contains n terms in increasing order, relatively prime to n, whose sum is a multiple of n and such that the row contains the smallest possible subset of consecutive numbers starting with 1.
%C A081521 The "smallest set of n distinct numbers" is not a well-defined term in the definition. Why is row 5 "1,2,3,6,8" but not "1,2,4,6,7"? Why is row 7 "1,2,3,4,5,8,12" but not "1,2,4,5,6,8,9"? - _R. J. Mathar_, Nov 12 2006
%e A081521 Triangle begins:
%e A081521   1;
%e A081521   1, 3;
%e A081521   1, 4, 7;
%e A081521   1, 3, 5,  7;
%e A081521   1, 2, 3,  6,  8;
%e A081521   1, 5, 7, 11, 13, 17;
%e A081521   1, 2, 3,  4,  5,  8, 12;
%e A081521   ...
%o A081521 (PARI) row(n) = {my(m=n*(n-1)/2, v); forstep(k=m+n/(2-n%2), oo, n, v=List([]); for(i=2, k-m, if(gcd(n, i)==1, listput(v, i))); if(#v>n-2, forsubset([#v, n-1], w, if(r=1+sum(i=1, n-1, v[w[i]])==k, return(concat(1, vector(n-1, i, v[w[i]]))))))); } \\ _Jinyuan Wang_, May 24 2020
%Y A081521 Cf. A081522, A081523, A081524.
%K A081521 nonn,tabl,more
%O A081521 1,3
%A A081521 _Amarnath Murthy_, Mar 27 2003
%E A081521 New definition proposed by _Omar E. Pol_, Mar 24 2008, in an attempt to answer _R. J. Mathar_'s questions.
%E A081521 Name corrected and more terms from _Jinyuan Wang_, May 24 2020