A226020 Composite squarefree numbers n such that the ratio (n + 1/2)/(p(i) + 1/2) is an integer, where p(i) are the prime factors of n.

13702, 42997, 1004062, 1684462, 38447662, 40243549, 70801087, 107728582, 409055062, 594021862, 760767262, 1045475437, 1104435202, 1471700587, 1686747562, 1920806662, 3136180162, 3469071937, 5291041297, 7239716347, 7903353667, 12738885862, 22711489762

Offset: 1

Author

Paolo P. Lava, May 23 2013

Keywords

• nonn
• hard

Comments

Also composite squarefree numbers n such that (2*p(i)+1) | (2*n+1).

Examples

			The prime factors of 13702 are 2, 13, 17 and 31. We see that (13702 + 1)/(2 + 1/2) = 5481, (13702 + 1/2)/(13 + 1/2) = 1015, (13702 + 1)/(17 + 1/2) = 783 and ( 13702 + 1/2)/(31 + 1/2) = 435. Hence 13702 is in the sequence.
The prime factors of 1123545 are 3, 5 and 74903. We see that
(1123545 + 1/2)/(3 + 1/2) = 321013, (1123545 + 1/2)/(5 + 1/2) = 204281 but (1123545 + 1/2)/(74903+ 1/2) = 321013/21401. Hence 1123545 is not in the sequence.
		

Links

• Giovanni Resta, Table of n, a(n) for n = 1..65 (terms < 3*10^12)

Crossrefs

Cf. A208728, A225702-A225720, A226111-A226114.

Programs

Maple

with(numtheory); A226020:=proc(i, j) local c, d, n, ok, p;
for n from 2 to i do if not isprime(n) then p:=ifactors(n)[2]; ok:=1;
for d from 1 to nops(p) do if p[d][2]>1 or not type((n+j)/(p[d][1]+j),integer) then ok:=0; break; fi; od;
if ok=1 then print(n); fi; fi; od; end: A226020(10^9,1/2);

Extensions

a(9)-a(23) from Giovanni Resta, Jun 02 2013