OEIS

The On-Line Encyclopedia of Integer Sequences® of Neil Sloane et al.

A Handbook of Integer Sequences, N. J. A. Sloane, Academic Press, New York and London, 1973.
The_Encyclopedia_of_Integer_Sequences, N. J. A. Sloane and S. Plouffe, Academic Press, San Diego, 1995, 587 pp. ISBN 0-12-558630-2.

Coincidences - search lists for sequences with common terms
Broken links detection and repair
WTM World of Terrel Trotter's Math (archive copy of the original webpage of 2004, c.f. his OEIS user page)
Engel expansion - English translation of Friedrich Engel's speech: Entwicklung der Zahlen nach Stammbrüchen. Verhandlungen der 52. Versammlung Deutscher Philologen und Schulmänner, 1913, Marburg, pp. 190-191
A220952: 0, 1, 2, 3, 4, 9, 14, 19, 18, 17, 16, 11, 12, 13, ..., 49 ... by Donald E. Knuth, Feb. 20, 2013, was unkn, with additional FASS curves: space-filling, self-avoiding, simple and self-similar curves
A131388, and A131393 with recovered Rule 2 and generalized negative-positive incrementing sequences of Clark Kimberling, with listing and program
A030707, A055187, A217760 and related sequences, listing and program for generalized cumulative counting, Clark Kimberling's problem no. 4
Triangles with interlacing rows, Clark Kimberling's problem no. 18; Perl and C programs find 1, 2, 20, 1744, 2 002 568, 42 263 042 752 triangles for the between condition

A003828, Numbers n such that n^4 is a primitive sum of 3 positive fourth powers: 422481, 2813001, 8707481, 12197457, 16003017, 16430513, 20615673, 44310257, 68711097, 117112081, 145087793, 156646737, 589845921, 638523249, 873822121, 1259768473, 1679142729, 1787882337, 1871713857
A035055, Kimberling's expulsion (shuffle) array visualization
A070165, Collatz conjecture: OEIS/3x+1_Problem (solution proposal)
A112273: 5, 15, 365, 945 - a puzzle sequence
- my guess: 5*3^0, 5*3^1, 5*73 (or should that be 315 = 5*7*3^2 ?), 5*7*3^3
A213457 Intertwining numbers: a(8) = 1058349286
A291939: 1, 12, 19, 27, 37 - overlapping of Collatz sequences, was unkn, with 3D visualization