OFFSET
1,1
COMMENTS
3991687693967 (13 digits) includes the longest (7 digits) palindromic prime suffix (7693967) among the left-truncatable primes (digit '0' excluded). The largest one (24 digits, see A253427) contains a nonprime palindrome of 7 digits (1264621). The terms from a(3) to a(13) cannot be written as a sum of 3 squares.
LINKS
Mikk Heidemaa, Related material (2015)
Eric Weisstein's World of Mathematics, Truncatable Prime
Wikipedia, Truncatable prime
FORMULA
a(n) = 3991687693967 mod 10^n for 1 <= n <= 13. - Mikk Heidemaa, Oct 07 2017
EXAMPLE
Triangular form:
----------------
............7
...........67
..........967*
.........3967
........93967
.......693967
......7693967**
.....87693967
....687693967
...1687693967
..91687693967
.991687693967
3991687693967***
----------------
* None from 3rd row (967,...,3991687693967) cannot be written as a sum of 3 squares.
** The palindromic prime suffix.
*** a^2 + b^2 + c^2 + d^2 + e^2 + f^2 + g * h * i = 3991687693967;
a=693967; b=93967; c=3967; d=967; e=67; f=7; g=4114278523; h=37; i=27.
a^2 + b^2 + c^2 + d^2 + e^2 + f^2 + g * h * i = 3991687693967^5;
a=693967; b=93967; c=3967; d=967; e=67; f=7;
g=3571123727278334614405609468109056139549629; h=228288322626423; i=124339.
(All primes.)
MATHEMATICA
Column[ Table[ Mod[ 3991687693967, 10^n], {n, 13}], Right] (* Mikk Heidemaa, Oct 07 2017 *)
CROSSREFS
KEYWORD
nonn,base,fini,full
AUTHOR
Mikk Heidemaa, Dec 31 2014
STATUS
approved