OFFSET
1,1
COMMENTS
Terms may be categorized as belonging to the following types:
type 1: products of 3 distinct primes p,q,r such that 2*p*q + 1 = r: 78, 406, 465, ... (27108 of the first 100000 terms);
type 2: products of 3 distinct primes p,q,r such that 2*p*q - 1 = r: 66, 190, 435, ... (26848 of the first 100000 terms);
type 3: products of 3 distinct primes p,q,r such that p*q + 1 = 2*r: 231, 561, 1653, ... (23050 of the first 100000 terms);
type 4: products of 3 distinct primes p,q,r such that p*q - 1 = 2*r: 105, 595, 741, ... (22983 of the first 100000 terms);
type 5: products of the cube of a prime p and a distinct prime q such that 2*p^3 + 1 = q: 136, 31375, 3544453, ... (6 of the first 100000 terms);
type 6: products of the cube of a prime p and a distinct prime q such that 2*p^3 - 1 = q: 1431, 1774977571, 12642646591, ... (4 of the first 100000 terms);
type 7: products of the cube of a prime p and a distinct prime q such that p^3 - 1 = 2*q: the only term of this type is 351 = 3^3 * 13.
(No term is a product of the cube of a prime p and a distinct prime q such that p^3 + 1 = 2*q.)
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
Type
(see
cmts) Initial terms Notes
----- ------------------------ -----------------------------
1 78, 406, 465, ... p*q*r such that 2*p*q + 1 = r
2 66, 190, 435, ... p*q*r such that 2*p*q - 1 = r
3 231, 561, 1653, ... p*q*r such that p*q + 1 = 2*r
4 105, 595, 741, ... p*q*r such that p*q - 1 = 2*r
5 136, 31375, 3544453, ... p^3*q such that 2*p^3 + 1 = q
6 1431, 1774977571, ... p^3*q such that 2*p^3 - 1 = q
7 351 (only) p^3*q such that p^3 - 1 = 2*q
MAPLE
select(t -> numtheory:-tau(t) = 8, [seq(i*(i+1)/2, i=1..1000)]); # Robert Israel, Jan 13 2020
MATHEMATICA
Select[PolygonalNumber@ Range[180], DivisorSigma[0, #] == 8 &] (* Michael De Vlieger, Jan 11 2020 *)
PROG
(PARI) isok(k) = ispolygonal(k, 3) && (numdiv(k) == 8); \\ Michel Marcus, Jan 11 2020
(Magma) [k:k in [1..16000]| IsSquare(8*k+1) and NumberOfDivisors(k) eq 8]; // Marius A. Burtea, Jan 12 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Jon E. Schoenfield, Jan 11 2020
STATUS
approved