OFFSET
1,1
COMMENTS
Numbers on the display of a 4-digit hh:mm digital clock that remain prime when the display mode is switched between 12-hour AM/PM and 24-hour time.
LINKS
Hugo Pfoertner, Table of n, a(n) for n = 1..108
Forums for the webcomic xkcd.com, 0247: "Factoring the Time", post by user jasticE, Apr 12 2007.
Randall Munroe, Factoring the time, xkcd Web Comic #247, Apr 11 2007.
EXAMPLE
a(49) = 1013 and a(103) = 2213 are in the sequence, because toggling the display mode of the digital clock will either leave 10:13 unchanged or switch between 22:13 and 10:13 at 10:13 PM. Both displayed times are prime when reading hh:mm as decimal number.
PROG
(PARI) apd(x)=2400*((x>1200)-1/2);
for(h=0, 23, for(m=0, 59, t=100*h+m; t12=t-apd(t); if(isprime(t)&&isprime(t12), print1(t, ", ")))) \\ Hugo Pfoertner, Jul 18 2018
CROSSREFS
KEYWORD
nonn,base,easy,fini,full
AUTHOR
Hugo Pfoertner, Jul 15 2018
STATUS
approved