OFFSET
1,2
COMMENTS
Numbers which, written as Roman numerals, are the same upside down and backwards.
Upside-down-invariant numbers are also called ambigrams. "Upside down" here means rotated by 180 degrees (i.e., central symmetry), NOT "vertically flipped" (symmetry w.r.t. horizontal line).
V, L, C, D, M (5, 50, 100, 500, 1000 in decimal) are not the same upside-down. Excludes "old style" Roman numeral representation of n (e.g., IIII rather than IV).
EXAMPLE
I, II, III, X, XIX, XX, XXX.
CROSSREFS
KEYWORD
nonn,base,fini,full
AUTHOR
Jonathan Vos Post, Aug 03 2012
STATUS
approved