A182781 Number of n-digit terms in A048398.

4, 4, 2, 1, 12, 20, 35, 28, 80, 114, 211, 228, 736, 1214, 2101, 2536, 7799, 13830, 22107, 27265, 82611, 144324, 259260, 354029, 901774, 1651718, 2913981, 3913728, 11048656, 19782855, 33483206, 49533124

Offset

1,1

Comments

Also, number of n-digit primes in A033075.

Appears to be strictly increasing for n >= 8. - Chai Wah Wu, May 31 2017

Links

Table of n, a(n) for n=1..32.

Maple

A182781aux := proc(Lhig, n) local lsb, a ; if n = 0 then if isprime(Lhig) then    1; else 0; end if; else a := 0 ; lsb := Lhig mod 10 ; if lsb > 0 then a := a + procname(10*Lhig+lsb-1, n-1) ; end if; if lsb < 9 then a := a + procname(10*Lhig+lsb+1, n-1) ; end if; a; end if; end proc:
A182781 := proc(n) if n = 1 then 4; else a := 0 ; for l from 1 to 9 do a := a + A182781aux(l, n-1) ; end do: a ; end if; end proc: # R. J. Mathar, Feb 01 2011

Prog

(Python 3.2 or higher)
from itertools import product, accumulate
from sympy import isprime
def A182781(n):
    if n == 1:
        return 4
    count = 0
    for d in [1, 3, 7, 9]:
        for elist in product([-1, 1], repeat=n-1):
            flist = [str(x) for x in accumulate([d]+list(elist)) if 0 <= x < 10]
            if len(flist) == n and flist[-1] != '0' and is_prime(int(''.join(flist[::-1]))):
                count += 1
    return count # Chai Wah Wu, Jun 05 2017

Crossrefs

Cf. A033075, A048398.

Sequence in context: A153163 A168455 A300153 * A291085 A193556 A120438

Adjacent sequences: A182778 A182779 A182780 * A182782 A182783 A182784

Keyword

nonn,base,more

Author

Zak Seidov, Feb 01 2011

Extensions

a(22)-a(24) from Chai Wah Wu, May 31 2017

a(25)-a(32) from Chai Wah Wu, Jun 05 2017

Status

approved