OFFSET
1,1
COMMENTS
Let p be prime; look for the smallest prime in {1|p|1, 3|p|3, 7|p|7, 9|p|9}, where '|' stands for concatenation; repeat until no such prime can be found; then height(p) = number of rows in pyramid.
a(13) > 10^10. - Donovan Johnson, Aug 13 2010
LINKS
H. Heinz, Patterns in Primes, Illustrating Two Prime Pyramids
Chai Wah Wu, On a conjecture regarding primality of numbers constructed from prepending and appending identical digits, arXiv:1503.08883 [math.NT], 2015.
EXAMPLE
Example for p=43: 43 3433 334333 93343339 3933433393 939334333939 39393343339393, stop; height(43)=7.
CROSSREFS
KEYWORD
nonn,nice,base,more
AUTHOR
Felice Russo, Jan 25 2000
EXTENSIONS
More terms from Naohiro Nomoto, Jul 14 2001
a(11)-a(12) from Donovan Johnson, Aug 13 2010
a(13) from Chai Wah Wu, Apr 10 2015
a(14)-a(15) from Giovanni Resta, May 15 2020
STATUS
approved