|
|
A215751
|
|
Numbers n such that tau(4n+2)=tau(4n)-2, where tau=A000005 gives the number of divisors.
|
|
1
|
|
|
3, 5, 8, 11, 23, 28, 29, 40, 41, 53, 83, 89, 92, 113, 124, 131, 164, 173, 175, 179, 188, 191, 192, 220, 233, 236, 239, 244, 251, 268, 281, 293, 316, 325, 356, 359, 419, 431, 443, 448, 452, 491, 507, 509, 593, 628, 641, 653, 659, 668, 683, 692, 719, 743, 747, 761, 764
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,1
|
|
COMMENTS
|
Motivated by the observation from A. Wesolowski that Sophie Germain primes A005384 satisfy this relation. A005384 is indeed exactly the subsequence of all primes in this sequence.
If p is an odd prime and 8*p+1 is in A006881, then 4*p is in the sequence. - Robert Israel, May 11 2016
|
|
LINKS
|
|
|
MAPLE
|
filter:= n -> numtheory:-tau(4*n+2)=numtheory:-tau(4*n)-2:
|
|
MATHEMATICA
|
Select[Range@ 800, DivisorSigma[0, 4 # + 2] == DivisorSigma[0, 4 #] - 2 &] (* Michael De Vlieger, May 12 2016 *)
|
|
PROG
|
(PARI) for(n=1, 999, numdiv(4*n+2)==numdiv(4*n)-2 & print1(n", "))
(Magma) [n: n in [1..764] | NumberOfDivisors(4*n+2) eq NumberOfDivisors(4*n)-2]; // Arkadiusz Wesolowski, May 11 2016
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|