This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A064509 #22 Jul 13 2025 17:08:58
%S A064509 2,3,5,7,10,13,15,17,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95,
%T A064509 100,105,110,115,120,125,130,135,140,145,150,155,160,165,170,175,180,
%U A064509 185,190,195,200,205,210,215,220,225,230,235,240,245,250,255
%N A064509 Marks (in fathoms) on lead line used by ships on the Mississippi River.
%C A064509 Certain depths have (or had) a visual and tactile indicator at positions on the lead line. All depths with such attachments are "marks". All others are "deeps." A leadsman measuring 12 feet of water calls "by the mark two (or twain)." If the depth on the lead is 36 feet (6 fathoms) he would call "by the deep six!".
%C A064509 Samuel Clemens chose the nom de plume Mark Twain because, for a riverboat skipper on the Mississippi, when the water was 12 feet deep, it was safe sailing for those boats.
%D A064509 Bowditch, The American Practical Navigator, 1931 edition.
%D A064509 Postings to newsgroup rec.org.sca, circa Oct 22, 1994 by djheydt(AT)uclink.berkeley.edu (Dorothy J. Heydt), Jeff Suzuki (jeffs(AT)math.bu.EDU) and Hal Ravn.
%H A064509 Paolo Xausa, <a href="/A064509/b064509.txt">Table of n, a(n) for n = 1..10000</a>
%H A064509 Mark S. Harris, <a href="http://www.florilegium.org/files/COMMERCE/measures-msg.text">Period measures and cautions for recipes</a>.
%H A064509 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).
%F A064509 For n >= 9, a(n) = 5(n-5).
%t A064509 LinearRecurrence[{2, -1}, {2, 3, 5, 7, 10, 13, 15, 17, 20, 25}, 100] (* _Paolo Xausa_, Jul 13 2025 *)
%K A064509 nonn,nice,easy
%O A064509 1,1
%A A064509 _Peter Shor_, Oct 06 2001
%E A064509 a(55)-a(56) corrected by _Sean A. Irvine_, Jul 14 2023