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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A074727 Number of steps needed to reach a prime when the following map is repeatedly applied to n: if n is even then 2n + SOD(n) + 1, otherwise 2n - SOD(n) - 1, where SOD(n) is the sum of the digits of n; or -1 if no prime is ever reached. 0
1, 1, 1, 2, 1, 2, 6, 7, 4, 1, 2, 4, 16, 1, 6, 6, 2, 3, 1, 3, 3, 6, 3, 5, 1, 2, 1, 2, 2, 2, 15, 1, 15, 1, 7, 3, 2, 21, 5, 15, 4, 16, 1, 8, 1, 7, 1, 2, 7, 7, 2, 1, 20, 2, 15, 1, 6, 1, 1, 8, 22, 2, 1, 20, 64, 3, 1, 31, 14, 22, 19, 66, 7, 1, 14, 1, 15, 10, 7, 2, 6, 19, 1, 4, 8, 2, 1, 7, 18, 3, 2, 1, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,4

COMMENTS

Records appear at 2, 5, 8, 9, 14, 39, 62, 66, 73, 98, 722, 22226, 23226, 38737, 55411, ....

Can it be proved that a(n) > 0 for all n > 1?

What is a(55411)?  If positive, it is greater than 35,000.

LINKS

Table of n, a(n) for n=2..94.

EXAMPLE

a(10) = 4 because 10 -> 22 -> 49 -> 84 -> 181.

CROSSREFS

Sequence in context: A025277 A248100 A153896 * A059587 A070236 A020825

Adjacent sequences:  A074724 A074725 A074726 * A074728 A074729 A074730

KEYWORD

nonn,base

AUTHOR

Jason Earls, Sep 04 2002

EXTENSIONS

Comments from Charles R Greathouse IV, Nov 18 2010.

a(55411) = 60796, assuming the 18,306-digit BPSW-probable prime is in fact prime.

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 August 20 10:05 EDT 2019. Contains 326149 sequences. (Running on oeis4.)