Search for a tool
Previous Prime Number

Tool to search for prime numbers located before a number, or the nth previous prime number smaller than/inferior to a given number.

Results

Previous Prime Number -

Tag(s) : Arithmetics

Share
Share
dCode and more

dCode is free and its tools are a valuable help in games, maths, geocaching, puzzles and problems to solve every day!
A suggestion ? a feedback ? a bug ? an idea ? Write to dCode!


Please, check our dCode Discord community for help requests!
NB: for encrypted messages, test our automatic cipher identifier!


Thanks to your feedback and relevant comments, dCode has developed the best 'Previous Prime Number' tool, so feel free to write! Thank you!

Previous Prime Number

Previous Prime Less than N Search



Answers to Questions (FAQ)

What is the previous prime number? (Definition)

For a given number $ N $, the previous prime number is the smallest number strictly less than $ N $ which is a prime number.

How to calculate the previous prime number?

No mathematical formula allow an automatic calculation of the prime number preceding any number N (less than N). dCode performs primality tests on numbers below the requested number until a prime number is found.

Example: The largest prime number that precedes 100 is 97

Example: The 10th prime number preceding 1000 is 937 (so there is a list of 9 prime numbers between 938 and 999 included)

The number $ N $ must be a positive real number superior to 3.

How to find greatest primes with n digits?

The largest number with $ n $ digits is $ 10^n -1 $ (that is always a multiple of 9), so the largest prime number with $ n $ digits is the prime number that precedes $ 10^n-1 $.

Example: The largest 4-digit number is $ 10^4 -1 = 10000-1 = 9999 $ so the largest prime number with $ n $ digits is the prime number preceding $ 10000 $ that is $ 9973 $.

The first 20 prime numbers in this list are: 7 (largest 1 digit prime), 97 (largest 2-digit prime), 997 (largest 3-digit prime), 9973, 99991, 999983, 9999991, 99999989, 999999937, 9999999967, 99999999977, 999999999989, 9999999999971, 99999999999973, 999999999999989, 9999999999999937, 99999999999999997, 999999999999999989, 9999999999999999961, 99999999999999999989 etc.

What are twin primes numbers?

A prime number and the previous prime number are twins primes when their difference equals 2.

Example: 17 and 19 are called twin primes as 19-17 = 2.

Source code

dCode retains ownership of the "Previous Prime Number" source code. Except explicit open source licence (indicated Creative Commons / free), the "Previous Prime Number" algorithm, the applet or snippet (converter, solver, encryption / decryption, encoding / decoding, ciphering / deciphering, translator), or the "Previous Prime Number" functions (calculate, convert, solve, decrypt / encrypt, decipher / cipher, decode / encode, translate) written in any informatic language (Python, Java, PHP, C#, Javascript, Matlab, etc.) and all data download, script, or API access for "Previous Prime Number" are not public, same for offline use on PC, mobile, tablet, iPhone or Android app!
Reminder : dCode is free to use.

Cite dCode

The copy-paste of the page "Previous Prime Number" or any of its results, is allowed as long as you cite dCode!
Cite as source (bibliography):
Previous Prime Number on dCode.fr [online website], retrieved on 2022-09-29, https://www.dcode.fr/previous-prime-number

Need Help ?

Please, check our dCode Discord community for help requests!
NB: for encrypted messages, test our automatic cipher identifier!

Questions / Comments

Thanks to your feedback and relevant comments, dCode has developed the best 'Previous Prime Number' tool, so feel free to write! Thank you!


https://www.dcode.fr/previous-prime-number
© 2022 dCode — The ultimate 'toolkit' to solve every games / riddles / geocaching / CTF.
 
Feedback