"Cuckoss's Egg" sequence, next number?

[robert Ihnot]

In The cuckoo's Egg by Cliff Stoll, an astronomer at Berkeley Lab and then at Harvard, who had discovered that hacker's infiltrated his computer--and of the military government. He was very good at understanding the hacker at at time when the goverment had never considered this possibility. He records a conversation with a friend at National Security Lab:

"OTTFFSS, what's the next letter Cliff?"

This was an oldie. "One, Two, Three....the next letter is E for eight."

"Complete the series Cliff: 1, 11, 21, 1211, 111221..."

Cliff adds, "I looked at it for five minutes and gave up. To this day, I still haven't solved it."

I really wonder about that series, does anyone see how it goes?

[Gokul43201]

Yes, this puzzle has been posted here (http://www.physicsforums.com/showthread.php?t=27453&highlight=1211)before :

In white Each new number tells the digits that occur in the previous number, in terms of their repetitions. Let me transcribe the existing series :

[ start with 1=ONE ]
11=one ONE
21=two ONEs
1211=one TWO, one ONE
111221=one ONE, one TWO & two ONEs

the next line must describe this last line, which has "three ONEs, two TWOs, & one ONE = 312211