A248047 a(1)=88; for n>=1, a(n+1) is the smallest palindromic 4-almost prime with a(n) as a central substring.

88, 1881, 218812, 12188121, 1121881211, 111218812111, 51112188121115, 1511121881211151, 815111218812111518, 78151112188121115187, 1781511121881211151871, 917815111218812111518719, 59178151112188121115187195, 9591781511121881211151871959

Offset

1,1

Comments

The 4-almost primes are the numbers that are the product of exactly four (not necessarily distinct) primes.

Links

Robert Israel, Table of n, a(n) for n = 1..36

Example

a(1)=88=2*2*2*11;
a(2)=1881=3*3*11*19;
a(3)=218812=2*2*11*4973.

Maple

rev:= proc(n) local L, i; L:= convert(n, base, 10); add(L[-i]*10^(i-1), i=1..nops(L)) end proc:
r:= 88: R:= r: dr:= 1+ilog10(r):
for count from 2 to 20 do
  for x from 1 do
      m:= ilog10(x)+1;
      y:= rev(x) + 10^m*(r + 10^dr*x);
      if NumberTheory:-Omega(y) = 4 then r:= y; R:= R, y; dr:= 1+ilog10(r); break fi;
od od;
R; # Robert Israel, Mar 18 2026

Mathematica

d[n_]:=IntegerDigits[n]; t = {x = 88}; Do[i = 1; While[!PrimeOmega[y = FromDigits[Flatten[{z = d[i], d[x], Reverse[z]}]]]==4, i++]; AppendTo[t, x = y], {n, 14}]; t

Crossrefs

Cf. A014613, A247483, A247484.

Sequence in context: A235018 A228810 A245954 * A107422 A210006 A194491

Adjacent sequences: A248044 A248045 A248046 * A248048 A248049 A248050

Keyword

nonn,base

Author

Michel Lagneau, Dec 01 2014

Status

approved