OFFSET
1,1
COMMENTS
Equivalently, a(n) is the sum of all but the last digit of the n-th prime, concatenated with that last digit.
It appears that as the prime number xyzd transformed by (x+y+z)*10 +d; the larger the prime the less frequent the result is prime....
LINKS
Shawn A. Broyles, Table of n, a(n) for n = 1..10000
FORMULA
Let ...xyzd represent the decimal expansion of prime(n); then a(n) = (... + x + y + z)*10 + d.
EXAMPLE
For p=1571 (prime), 1+5+7 = 13; 13*10 = 130; 130+1 = 131 (prime).
MAPLE
map(t -> 10*convert(convert(t, base, 10), `+`)-9*(t mod 10), [seq(ithprime(i), i=1..100)]); # Robert Israel, Mar 25 2018
MATHEMATICA
Array[10 Total@ # - 9 Last@ # &@ IntegerDigits[Prime@ #] &, 67] (* Michael De Vlieger, Apr 27 2018 *)
PROG
(PARI) a(n) = my(p=prime(n); d=p % 10); sumdigits(p-d)*10+d; \\ Michel Marcus, Mar 23 2018
CROSSREFS
KEYWORD
nonn,easy,base,less
AUTHOR
Edmund Algeo, Mar 19 2018
STATUS
approved