login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A288212 Start with 2n, and if 2n+1 is composite, multiply by the smallest factor of 2n+1; if that number plus 1 is composite, multiply by the smallest factor of that number plus 1; continue until you have a number where z+1 is prime; this is the final z+1, or 0 if sequence diverges. 0
3, 5, 7, 1321, 11, 13, 43, 17, 19, 61, 23, 1321, 79, 29, 31, 97, 3571, 37, 571, 41, 43, 4621, 47, 337, 151, 53, 271, 21217561, 59, 61, 47059, 29761, 67, 1021, 71, 73, 223, 6917, 79, 241, 83, 421, 6221671, 89, 631, 277, 23971, 97, 1471, 101, 103, 313, 107, 109, 331 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

a(126) is unknown; it's known to either be 0 or have more than 152 digits.

LINKS

Table of n, a(n) for n=1..55.

EXAMPLE

a(17) = 3571 because:

34+1=35 is composite, smallest prime factor 5, 34*5=170

170+1=171 is composite, smallest prime factor 3, 170*3=510

510+1=511 is composite, smallest prime factor 7, 510*7=3570

3570+1=3571 is prime.

PROG

(PARI) a(n) = {my(x = 2*n); while (! isprime(x+1), x = x*vecmin(factor(x+1)[, 1]); ); x+1; } \\ Michel Marcus, Jun 07 2017

CROSSREFS

Cf. A152466 (sequence that if it terminates with a number a where a+1 is prime, the final prime is a(126) of this sequence.)

Sequence in context: A065824 A191546 A069463 * A273010 A073691 A110336

Adjacent sequences:  A288209 A288210 A288211 * A288213 A288214 A288215

KEYWORD

nonn

AUTHOR

J. Lowell, Jun 06 2017

EXTENSIONS

More terms from Michel Marcus, Jun 07 2017

STATUS

approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 19 17:48 EDT 2019. Contains 324222 sequences. (Running on oeis4.)