A321687 a(n) = (2*n+1)-digit palindrome formed by first concatenating A158289(i) for i=1..n+1 and then A158289(n-j) for j=1..n.

1, 121, 12321, 1234321, 123454321, 12345654321, 1234567654321, 123456787654321, 12345678987654321, 1234567898987654321, 123456789878987654321, 12345678987678987654321, 1234567898765678987654321, 123456789876545678987654321, 12345678987654345678987654321

Offset: 0

Author

Felix Fröhlich, Nov 17 2018

Keywords

• nonn
• base
• easy

Examples

			                     1
                    121
                   12321
                  1234321
                 123454321
                12345654321
               1234567654321
              123456787654321
             12345678987654321
            1234567898987654321
           123456789878987654321
          12345678987678987654321
         1234567898765678987654321
        123456789876545678987654321
       12345678987654345678987654321
      1234567898765432345678987654321
     123456789876543212345678987654321
    12345678987654321012345678987654321
   1234567898765432101012345678987654321
  123456789876543210121012345678987654321
		

Crossrefs

Cf. A158289.

Programs

PARI

a158289(n)=abs(n-round(n/18)*18)
eva(n) = subst(Pol(n), x, 10)
a(n) = if(n==0, return(1)); my(v=vector(2*n+1), x=#v); for(k=1, ceil(#v/2), v[k]=a158289(k)); while(1, v[x]=v[#v-x+1]; x--; if(x==ceil(#v/2), return(eva(v))))