A342927 The smallest polygonality of numbers that have exactly two different representations as polygonal numbers (A177029).

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

Author

Michel Marcus, Mar 29 2021

Keywords

• nonn

Examples

			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.
		

Links

• Michel Marcus, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A177028, A177029, A342928.

Programs

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], ", ")););}