OFFSET
1,1
COMMENTS
Start with a palindromic prime p; look for smallest palindromic prime that has previous term as a centered substring and has 2 more digits (i.e., one more digit at each end); repeat until no such palindromic prime can be found; then height(p) = number of rows in pyramid. Each row of pyramid must be the smallest prime that can be used. Then a(n) = smallest value of p that generates a pyramid of height n.
LINKS
G. L. Honaker, Jr. and Chris K. Caldwell, Palindromic prime pyramids
Ivars Peterson's MathTrek, Primes, Palindromes, and 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
a(1) = 11.
a(4) = 5:
5
151
31513
3315133, stop;
height(5)=4.
a(6)=16362:
16361
1163611
311636113
33116361133
3331163611333
333311636113333, stop;
height(16361)=6.
CROSSREFS
KEYWORD
nonn,base,more,changed
AUTHOR
Felice Russo, Jan 25 2000
EXTENSIONS
Added a(10)-a(11) and corrected a(4) - Chai Wah Wu, Apr 09 2015
Entry revised by N. J. A. Sloane, Apr 13 2015
a(12) from Michael S. Branicky, Oct 28 2024
STATUS
approved