login
A249264
Sequence of distinct least nonnegative numbers such that the average of the first n terms is a triangular number.
1
0, 2, 1, 9, 3, 21, 6, 38, 10, 60, 15, 87, 112, 28, 148, 36, 189, 45, 235, 55, 286, 66, 342, 78, 403, 91, 469, 105, 540, 120, 616, 136, 697, 153, 783, 171, 874, 190, 970, 210, 1071, 231, 1177, 253, 1288, 276, 1404, 300, 1525, 325, 1651, 351, 1782, 378, 1918, 406, 2059, 435, 2205, 465
OFFSET
1,2
COMMENTS
Similar to A248983 except a(1) = 0, the zeroth triangular number.
Note that the sum of the first 12 terms is 252. Also, one can show that (252+sum_{i=7..n}(A000566(n)+A000217(n)))/(2*n) = n*(n+1)/2. So, for n > 6, if a(2*k-1) = A000566(k) and a(2*k-2) = A000217(k) for all 6 < k <= n, then a(2*n) = n*(n+1)/2.
Similarly, for n > 6, if a(2*k-1) = A000566(k) and a(2*k) = A000217(k) for all 6 < k <= n, then a(2*n+1) = A000566(n).
FORMULA
Empirical g.f.: x^2*(66*x^16-66*x^15-153*x^14+153*x^13+91*x^12-91*x^11-3*x^2-x-2) / ((x-1)^3*(x+1)^3). - Colin Barker, Oct 24 2014
Conjectured: For n > 6, a(2*n-1) = A000566(n) and a(2*n) = A000217(n).
PROG
(PARI) v=[]; n=0; while(n<5000, num=(vecsum(v)+n); if(num%(#v+1)==0&&vecsearch(vecsort(v), n)==0, for(i=0, n, if(i*(i+1)/2>(num/(#v+1)), break); if(i*(i+1)/2==(num/(#v+1)), print1(n, ", "); v=concat(v, n); n=0; break))); n++)
CROSSREFS
Sequence in context: A298738 A124776 A099285 * A188108 A166890 A197411
KEYWORD
nonn
AUTHOR
Derek Orr, Oct 23 2014
STATUS
approved