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

1,1

Links

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

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

Cf. A177028, A177029, A342928.

Sequence in context: A121889 A332026 A205692 * A363013 A226798 A045997

Adjacent sequences: A342924 A342925 A342926 * A342928 A342929 A342930

Keyword

nonn

Author

Michel Marcus, Mar 29 2021

Status

approved