login
A342927
The smallest polygonality of numbers that have exactly two different representations as polygonal numbers (A177029).
2
3, 4, 3, 5, 4, 7, 5, 9, 4, 10, 11, 12, 7, 5, 14, 8, 15, 9, 17, 4, 10, 19, 20, 11, 21, 22, 8, 24, 25, 14, 15, 29, 10, 30, 16, 31, 5, 32, 17, 11, 34, 35, 19, 37, 39, 13, 21, 4, 42, 22, 14, 44, 23, 45, 8, 47, 25, 50, 51, 17, 54, 28, 55, 29, 57, 4, 30, 59, 60, 31
OFFSET
1,1
LINKS
EXAMPLE
6 is A177029(1); it is a 3-gonal and 6-gonal number; so a(1) = 3.
9 is A177029(2); it is a 4-gonal and 9-gonal number; so a(2) = 4.
PROG
(PARI) row(n) = my(v=List()); fordiv(2*n, k, if(k<2, next); if(k==n, break); my(s=(2*n/k-4+2*k)/(k-1)); if(denominator(s)==1, listput(v, s))); Vecrev(v); \\ A177028
lista(nn) = {for (n=3, nn, my(r = row(n)); if (#r == 2, print1(r[1], ", ")); ); }
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Mar 29 2021
STATUS
approved