login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A307546 Prime indices of A195685. 1
7, 14, 15, 20, 26, 28, 29, 36, 45, 48, 66, 70, 89, 98, 104, 115, 122, 126, 142, 152, 157, 161, 164, 167, 177, 182, 186, 191, 194, 199, 202, 205, 216, 218, 219, 244, 264, 279, 283, 295, 297, 299, 324, 329, 336, 342, 362, 408, 416, 423, 430, 440, 457, 498, 500 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Since only the indices are listed, this sequence is more compact than A195685. As described there, all elements of the triples (2*prime(a(n))-1, 2*prime(a(n)), 2*prime(a(n))+1) are products of exactly two distinct primes. Such values are called "squarefree semiprimes" in A006881.
LINKS
FORMULA
a(n) = A000720(A195685(n)).
A195685(n) = A000040(a(n)).
EXAMPLE
A000040(a(1)) = 17 = A195685(1).
A000040(a(5)) = A000040(26) = 101 = A195685(5).
MAPLE
with(numtheory):
q:= n-> (p-> tau(2*p-1)=4 and tau(2*p+1)=4)(ithprime(n)):
select(q, [$1..500])[]; # Alois P. Heinz, Apr 18 2019
PROG
(PARI) isok(k) = my(p=prime(k)); (numdiv(2*p-1) == 4) && (numdiv(2*p+1) == 4); \\ Michel Marcus, Nov 30 2022
CROSSREFS
Sequence in context: A354791 A107976 A022557 * A297261 A296705 A297138
KEYWORD
nonn
AUTHOR
Peter Dolland, Apr 14 2019
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified July 20 09:46 EDT 2024. Contains 374445 sequences. (Running on oeis4.)