OFFSET
1,2
COMMENTS
a(n) also equals the smallest positive integer such that d(a(n)(a(n)+1)) > d(a(n-1)(a(n-1)+1)).
That is, this is the sequence of indices of oblong numbers that have more divisors than the preceding oblong numbers. - Michel Marcus, Jul 13 2019
EXAMPLE
Since a(7) = 15, we want for a(8) the smallest positive integer m such that d(m)*d(m+1) > d(15)d(16) = 4*5=20. Checking: d(16)*d(17)=10, d(17)*d(18)=12, d(18)*d(19)=12, d(19)*d(20)=12. All of these are <= 20. But d(20)*d(21) = 6*4=24, which is > 20. So a(8) = 20.
PROG
(PARI) lista(nn) = {my(m = 0, nm); for (n=1, nn, if ((nm = numdiv(n*(n+1))) > m, m = nm; print1(n, ", ")); ); } \\ Michel Marcus, Jul 13 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Leroy Quet, Jul 06 2008
EXTENSIONS
More terms from Max Alekseyev, Apr 26 2010
STATUS
approved