Liquidity and Capital Resources
We fund our operations primarily through cash generated from operations. As of December 31, 2019, we had cash and cash equivalents totalling $36.6 million, marketable securities of $434.8 million and accounts receivable of $97.9 million.
We believe our existing cash balances and anticipated cash flow from future operations will be sufficient to meet our working capital and capital expenditure needs for at least the next 12 months and the foreseeable future. Our future capital requirements may vary materially from those currently planned and will depend on many factors, including our rate of revenue growth, the timing and extent of spending on research and development efforts and other business initiatives, purchases of capital equipment to support our growth, the expansion of sales and marketing activities, any expansion of our business through acquisitions of or investments in complementary products, technologies or businesses, the use of working capital to purchase additional inventory, the timing of new product introductions, market acceptance of our products and overall economic conditions. To the extent that current and anticipated future sources of liquidity are insufficient to fund our future business activities and requirements, we may be required to seek additional equity or debt financing. In the event additional financing is required from outside sources, we may not be able to raise it on terms acceptable to us or at all. We believe our existing cash balances and anticipated cash flow from future operations will be sufficient to meet our working capital and capital expenditure needs for at least the next 12 months and the foreseeable future. Our future capital requirements may vary materially from those currently planned and will depend on many factors, including our rate of revenue growth, the timing and extent of spending on research and development efforts and other business initiatives, purchases of capital equipment to support our growth, the expansion of sales and marketing activities, any expansion of our business through acquisitions of or investments in complementary products, technologies or businesses, the use of working capital to purchase additional inventory, the timing of new product introductions, market acceptance of our products and overall economic conditions. To the extent that current and anticipated future sources of liquidity are insufficient to fund our future business activities and requirements, we may be required to seek additional equity or debt financing. In the event additional financing is required from outside sources, we may not be able to raise it on terms acceptable to us or at all. We believe our existing cash balances and anticipated cash flow from future operations will be sufficient to meet our working capital and capital expenditure needs for at least the next 12 months and the foreseeable future. Our future capital requirements may vary materially from those currently planned and will depend on many factors, including our rate of revenue growth, the timing and extent of spending on research and development efforts and other business initiatives, purchases of capital equipment to support our growth, the expansion of sales and marketing activities, any expansion of our business through acquisitions of or investments in complementary products, technologies or businesses, the use of working capital to purchase additional inventory, the timing of new product introductions, market acceptance of our products and overall economic conditions. To the extent that current and anticipated future sources of liquidity are insufficient to fund our future business activities and requirements, we may be required to seek additional equity or debt financing. In the event additional financing is required from outside sources, we may not be able to raise it on terms acceptable to us or at all.
On April 30, 2018, our Board of Directors authorized a stock repurchase program for the repurchase of up to $60.0 million of our common stock, of which $39.7 million was used to repurchase shares in 2018 prior to the program’s expiration on December 31, 2018.
Table:
 |  | Year Ended December 31, | 
 | 2019 | 2018 | 2017
 |  | (in thousands) | 
Cash and cash equivalents | $36,617 | $60,444 | $67,495
Marketable securities | 434,761 | 339,424 | 297,115
Working capital | 368,912 | 370,445 | 361,621
Net cash provided by operating activities | 72,819 | 83,085 | 61,893
Net cash used in investing activities | (103,579) | (56,237) | (207,907)
Net cash provided by (used in) financing activities | 6,933 | (33,899) | 5,477


Question: What are the 3 factors that affect future capital requirement?
Answer: rate of revenue growth, the timing and extent of spending on research and development efforts and other business initiatives, purchases of capital equipment to support our growth, the expansion of sales and marketing activities, any expansion of our business through acquisitions of or investments in complementary products, technologies or businesses, the use of working capital to purchase additional inventory, the timing of new product introductions, market acceptance of our products and overall economic conditions.