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A272617
Concatenation of the numbers from n down to 1 with numbers from 1 to n.
1
11, 2112, 321123, 43211234, 5432112345, 654321123456, 76543211234567, 8765432112345678, 987654321123456789, 1098765432112345678910, 11109876543211234567891011, 121110987654321123456789101112, 1312111098765432112345678910111213, 14131211109876543211234567891011121314
OFFSET
1,1
COMMENTS
Conjecture: a(1) is the only prime number.
No other prime terms up to a(8000). - Giovanni Resta, May 07 2016
LINKS
FORMULA
a(n) = A000422(n) concatenated with A007908(n).
EXAMPLE
a(1) = concatenate("1", "1") = 11.
a(2) = concatenate("2", "1", "1", "2") = 2112.
a(3) = concatenate("3", "2", "1", "1", "2", "3") = 321123.
MATHEMATICA
FromDigits@Flatten@IntegerDigits@Join[Reverse@#, #] & /@ Table[Range@n, {n, 20}]
PROG
(PARI) a(n)={fromdigits(concat(concat(Vecrev(vector(n, i, digits(i)))), concat(vector(n, i, digits(i)))))} \\ Andrew Howroyd, Dec 23 2019
CROSSREFS
Sequence in context: A004822 A265876 A078271 * A301473 A087403 A085878
KEYWORD
nonn,base
AUTHOR
EXTENSIONS
Terms a(11) and beyond from Andrew Howroyd, Dec 23 2019
STATUS
approved