OFFSET
1,2
COMMENTS
Also, numbers whose decimal digits (d[0], ..., d[n]) are such that for all k = 0..n, d[k] + d[n-k] = 0 (mod 10). In particular, if the number of digits n+1 is odd, the middle digit must be either 5 or 0.
The variant A299539 is obtained by excluding terms with a digit 0, i.e., removing all terms that are in A011540, or taking intersection with zeroless numbers A052382. - M. F. Hasler, Nov 25 2020
EXAMPLE
The Morse code for digits is "-----" for 0, ".----" for 1, "..---" for 2, ..., "....." for 5, "-...." for 6, ..., "----." for 9. (In A060109 a dot is coded with a digit 1 and a dash with a digit 2.)
We see that 0 and 5 are the only digits with palindromic Morse code, this yields a(1) and a(2).
Two digit numbers must be of the form 10*a + (10-a), with a = 1, ..., 9, in order to have palindromic Morse code. This yields the 9 terms a(3), ..., a(11).
Three-digit terms must have 0 or 5 as middle digit and yield a two-digit term when that middle digit is deleted: this yields the next 18 terms a(12 .. 29).
MATHEMATICA
With[{a = Association@ Array[# -> If[# < 6, PadRight[ConstantArray[1, #], 5, 2], PadRight[ConstantArray[2, # - 5], 5, 1]] &, 10, 0]}, Select[Range[0, 3007], PalindromeQ[Flatten@ Riffle[Map[Lookup[a, #] &, IntegerDigits[#]], 0]] &]] (* Michael De Vlieger, Nov 02 2020 *)
PROG
(PARI) select( is(n)=(Vecrev(n=digits(n))+n)%10==0, [0..3333])
CROSSREFS
KEYWORD
nonn,base
AUTHOR
M. F. Hasler, Nov 02 2020
STATUS
approved