A090268 Smallest prime obtained by sandwiching prime(n) between identical digits, except that a(5) = 0.

727, 131, 151, 373, 0, 331333, 1171, 9199, 1231, 1291, 3313, 3373, 3413, 3433, 1471, 1531, 3593, 3613, 3673, 7717, 3733, 3793, 1831, 1111891111, 11197111, 31013, 31033, 11071, 91099, 11131, 3312733, 11311, 111137111, 31393, 11491, 31513

Offset

1,1

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Maple

isA010785 := proc(n) convert(convert(n, base, 10), set) ; if nops(%) = 1 then true ; else false ; fi ; end: A055642 := proc(n) ilog10(n)+ 1; end: A090268 := proc(n) local kern, k, p ; if n = 5 then RETURN(0) ; fi ; kern := ithprime(n) ; k := 1 ; while true do if isA010785(k) then p := k+10^A055642(k)*kern+k*10^(A055642(k)+A055642(kern)) ; if isprime(p) then RETURN(p) ; fi ; fi ; k := k+1 ; od ; end: seq(A090268(n), n=1..40); # R. J. Mathar, Jul 20 2007

Prog

(Python)
from sympy import isprime, prime
def a(n):
    if n == 5: return 0
    spn, digits = str(prime(n)), 1
    while True:
        for sk in [d*digits for d in "1379"]:
            if isprime(t:=int(sk + spn + sk)):
                return t
        digits += 1
print([a(n) for n in range(1, 37)]) # Michael S. Branicky, Jun 13 2026

Crossrefs

Cf. A090269.

Sequence in context: A185527 A306450 A259237 * A090267 A084412 A068688

Adjacent sequences: A090265 A090266 A090267 * A090269 A090270 A090271

Keyword

base,nonn

Author

Amarnath Murthy, Nov 28 2003

Extensions

More terms from R. J. Mathar, Jul 20 2007

Status

approved