page_title: HP-65
section_title: Significant applications
hierarchical_section_title: HP-65 / Significant applications
caption_attribution_description: English: HP65 display after result of test programm from card reader
context_page_description: The HP-65 is the first magnetic card-programmable handheld calculator. Introduced by Hewlett-Packard in 1974 at an MSRP of $795, it featured nine storage registers and room for 100 keystroke instructions. It also included a magnetic card reader/writer to save and load programs. Like all Hewlett-Packard calculators of the era and most since, the HP-65 used Reverse Polish Notation and a four-level automatic operand stack.
  Bill Hewlett's design requirement was that the calculator should fit in his shirt pocket. That is one reason for the tapered depth of the calculator. The magnetic program cards are fed in at the thick end of the calculator under the LED display. The documentation for the programs in the calculator is very complete, including algorithms for hundreds of applications, including the solutions of differential equations, stock price estimation, statistics, and so forth.
context_section_description: During the 1975 Apollo-Soyuz Test Project, the HP-65 became the first programmable handheld calculator in outer space. It was carried as a backup in case of a problem with the Apollo Guidance Computer. It was not put into operation, though, as the Apollo computer did not malfunction during that mission.
  Also in 1975, Mitchell Feigenbaum, using the small HP-65 calculator he had been issued at the Los Alamos National Laboratory, discovered that the ratio of the difference between the values at which successive period-doubling bifurcations occur tends to a constant of around 4.6692... This "ratio of convergence" is now known as the first Feigenbaum constant.
caption: HP-65 display after result of test program from card reader
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