# Name

smallprime - find a number that can be expressed as a product of powers of small prime numbers such as 2,3,and 5. The output number is smaller while close to the input number. This function gives the image size that is in favour of applying FFT transformation( We call it FFT-friendly).

# Usage

num_fft = smallprime(arbit_num, numprime)

## Input

- arbit_num
- an arbitary integer number
- numprime
- the number of prime number in the product.

## Output

- num_fft
- a number close to while less than arbit_num. It is equal to a product of powers of small primes (such as 2, 3, 5, 7,... ) only.

## Options

- None

# Description

- 1. The recommended number of primes is 3, which means the num_fft value can be a product of 2, 3, 5.
``"num_fft"(le\"arbit_num")=2^k*3^l*5^m...``- 2. It is highly recommended for the user to apply this function to determine the image size of protein particle during the initial stage of performing single particle 3D reconstruction.
- 3. When applying Fourier Filter to large micrographs(particular those micrographs scanned from films), it is highly recommended to window micrographs using this FFT-friendly image size calculated by this function.

# Reference

- None

# Author / Maintainer

- Zhong Huang

# Keywords

- category 1
- UTILITIES

# Files

- fundamentals.py

# Maturity

- stable
- works for most people, has been tested; test cases/examples available.

# Bugs

None. It is perfect.