OFFSET
3,1
COMMENTS
The corresponding indices of n-gonal pyramidal numbers are 6, 7, 20, 79, 90, 203, ...
LINKS
Eric Weisstein's World of Mathematics, Pyramidal Number
EXAMPLE
a(4) = 140, because 140 is a square pyramidal number that has 4 square pyramidal divisors {1, 5, 14, 140} and this is the smallest such number.
PROG
(PARI)
pyramidal(k, r)=(k*(k+1)*((r-2)*k + (5-r)))\6;
ispyramidal(n, r) = pyramidal(sqrtnint(6*n\(r-2) + sqrtnint(n, 3), 3), r) == n;
a(n) = if(n<3, return()); for(k=1, oo, my(t=pyramidal(k, n)); if(sumdiv(t, d, ispyramidal(d, n)) == n, return(t))); \\ Daniel Suteu, Dec 06 2022
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Ilya Gutkovskiy, Dec 03 2022
EXTENSIONS
a(9)-a(17) from Daniel Suteu, Dec 06 2022
STATUS
approved