octodecimal

The decimal scheme should have been made a duodecimal, or better a quinquedecimal or octodecimal one; the expansive system would need a more than India-rubber elasticity to include all the whimsical creations of an author's brain.

1907, The Dial - Volume 43, page 411

By studying other number system such as unodecimal (base-11), duodecimal (base-12), tridecimal (base-13), quadrodecimal (base-14), pentadecimal (base-15), heptadecimal (base-17), octodecimal (base-18), nonadecimal (base-19) and vigesimal (base-20), researcher will gain a better understanding of how number systems work in general.

2012 January, Debasis Das, U.A. Lanjewar, “Realistic Approach of Strange Number System from Unodecimal to Vigesimal”, in International Journal of Computer Science and Telecommunications, volume 3, number 1

Traditional number system — binary, octal, decimal and hexadecimal, in regular use and Strange number system, with an extra edge of memorized information with greater density, zero redundancy problem, avoiding sign problem and reducing complexity of interconnections, - unodecimal, duodecimal, tridecimal, quadrodecimal, pentadecimal, heptadecimal, octodecimal, nona decimal, vigesimal and further are discussed through compliments.

2013 May, “Subtraction in Traditional and Strange Number System by r's and r-1's Compliments”, in International Journal of Computer Applications, volume 70, number 23

I say very considerably greater than, because only a certain number of the terms which satisfy the required conditions of order and weight actually appear in the octodecimal invariant in question.

1879, The Messenger of Mathematics, page 7

18 modes of archery and use of other implements of war (Adavuyal and Ayudhet prayogangal), 18 kinds of manners and customs (Nattacharangal) and there seems to be something of importance in this octodecimal division.

1950, Ramananda Chatterjee, The Modern Review - Volume 87, page 404

Of course all symmetrical invariants have an invariable sign ; but this is not the case with skew invariants, as for example, M. Hermite's octodecimal invariant of a binary quintic, which will change its sign with that of the determinant of transformation.

1973, James Joseph Sylvester, 1854-1973, page 387

But this magnificent discovery, whereby the determination of the number of fundamental invariants to a binary quantic of a given degree is reduced to a problem in the partition of numbers, it is but justice to M. Hermite to state, took its rise in that great analyst's discovery of the octodecimal invariant of the binary quintic.

2012, James Joseph Sylvester, H. F. Baker, The Collected Mathematical Papers of James Joseph Sylvester