A384016 a(n) is the number of n-digit terms in A384015.

0, 1, 0, 0, 74, 673, 5851, 50977, 451608, 4048657, 36675547, 335269867, 3087739250, 28615970101, 266630103368, 2495975596632

Offset

1,5

Links

Table of n, a(n) for n=1..16.

Mathematica

a[1]:=0; a[n_]:=Module[{count=0}, For[k=Prime[PrimePi[10^(n-1)]+1], k<=Prime[PrimePi[10^n-1]], k=NextPrime[k], If[Part[d=IntegerDigits[k], l=IntegerLength[k]]==Part[d, l-1] &&Part[d, 1]==Part[d, 2], count++]]; count]; Array[a, 7]

Prog

(Python)
from sympy import isprime
def a(n):
    if n < 3: return n-1
    return sum(1 for i in range(1, 10) for j in range(i*11*10**(n-2), (i*11+1)*10**(n-2), 100) for k in (11, 33, 77, 99) if isprime(j+k))
print([a(n) for n in range(1, 10)]) # Michael S. Branicky, May 19 2025

Crossrefs

Cf. A383979, A384014, A384015.

Sequence in context: A205584 A280402 A250913 * A186124 A204353 A300487

Adjacent sequences: A384013 A384014 A384015 * A384017 A384018 A384019

Keyword

nonn,base,more

Author

Stefano Spezia, May 17 2025

Extensions

a(11) from David Radcliffe, May 18 2025

a(12)-a(13) from Michael S. Branicky, May 19 2025

a(14) from Michael S. Branicky, May 22 2025

a(15) from Lyle Blosser, Aug 24 2025

a(16) from Lyle Blosser, Aug 30 2025

Status

approved