OFFSET
1,1
COMMENTS
Number of primes of the form: ...d3d2d1d0 = d0 * 10 ^ 0 + d1 * 10 ^ 1 + d2 * 10 ^ 2 + d3 * 10 ^ 3 + ... where d0, d1, d2, d3, ... are primes with one digit. The i-th element of the sequence is the number of primes with i digits. Approximation of the sum of the sequence up to a(k) for large values of k: Sum_{i=1..k} a(i) = 10^k / (k*log(10))*(2/5)^(k-1).
EXAMPLE
a(4) = 38 because there are 38 numbers of the form d3d2d1d0 with d0, d1, d2, d3 prime numbers, namely, 2237, 2273, 2333, 2357, 2377, 2557, ..., 7753, 7757.
CROSSREFS
KEYWORD
nonn,base,more
AUTHOR
Giorgio Balzarotti & Paolo P. Lava, Dec 07 2007
EXTENSIONS
a(12)-a(14) from Donovan Johnson, Feb 05 2010
a(15)-a(16) from Chai Wah Wu, Nov 28 2015
a(17)-a(18) from Giovanni Resta, May 25 2016
STATUS
approved